Suli are an elemental race like Ifrits and the like, but the difference is that they have ties to ALL FOUR elements, and so they can be very versatile with their use of elemental abilities. They're uncommon creatures, though (and not legal for PFS play) so there are fewer chances to make use of their unique skills.
Ability Scores: +2 Strength, +2 Charisma, -2 Intelligence. A combination of Strength and Charisma bonuses is relatively rare, and it makes Sulis perfect as Paladins and battle Oracles, and you can also put together an excellent Dragon Disciple build with them!
Type: Sulis are outsiders with the native subtype, so like the other elemental races they they can’t be affected by spells and effects that target humanoids, such as enlarge person and charm person, so this is a double-edged sword..
Size: Sulis are Medium size, just like humans.
Speed: Sulis move the same speed as humans.
Energy Resistance: Sulis have energy resistance 5 to ALL FOUR elements, which is just incredible for anyone.
Negotiator: Sulis get a +2 bonus to Diplomacy and Sense Motive checks, which can come in very handy in social situations. Add to that their racial bonus to Charisma and most Sulis will be very good at negotiations.
Elemental Assault: This is an interesting ability that lets you add +1d6 energy damage of any of the four types to your unarmed strikes or any weapons held (even ranged ones!) for one round per character level. Obviously this will be much more useful for martial characters, but even spellcasters can make use of it.
Low-light Vision: As with all the other races that have low-light, this is just not as useful as darkvision would be, but it can come in handy in some situations.
Alternate Racial Traits:
Energy Strike [Elemental Assault, Energy Resistance]: Elemental Assault is an amazing ability, and this makes your Elemental Assault even better, HOWEVER you lose resistance to the other three elements, so I really have to say that this one it's worth the trade-off unless you have a specific plan for making use of the extra ability you get when using your Elemental Assault power.
Alchemist: A penalty to Intelligence is a bad thing for an Alchemist, but you can still make a decent melee combatant with lots of natural attacks thanks to Feral Mutagen, and make good use of the Elemental Assault racial ability.
Barbarian: A bonus to Strength is always helpful for a Barbarian, and this is another class that can make excellent use of Elemental Assault by getting a lot of natural or weapon attacks when raging.
Bard: The Charisma bonus is great, but most Bards like to have high Intelligence too for the skill points, and this is a case where you don't. You do have Negotiator though, so this is a pretty good choice overall.
Cavalier [Samurai]: Strength and Charisma bonuses both synergize well with the Cavalier class, so this one should work very well. Take advantage of Elemental Assault by activating it just before you charge with a lance to double that damage!
Cleric: You don't really have anything going for or against you with a Cleric build, so go for it if you like!
Druid: Same as the Cleric, there's nothing saying you can't make a great Druid, but this build doesn't have anything specifically going for it either.
Fighter: +2 Strength is great for a melee fighter, and you'll be able to make excellent use of Elemental Assault with whatever weapon you specialize in. Since you have a lot of feats available, consider grabbing Incremental Elemental Assault and Extra Elemental Assault to get the most possible use out of the ability.
Gunslinger: No bonuses to either Dexterity or Wisdom means you're not going to be the best Gunslinger out there, but there's also no real reason that you can't make a decent Gunslinger if you try hard enough. With the Mysterious Stranger archetype, this will work better, but losing Quick Clear is a huge issue for many players. Make sure to upgrade your gun with the Reliable property as soon as possible to keep those misfires from happening.
Inquisitor: Inquisitors generally like to have a lot of skills, but you can get away with fewer thanks to the Monster Lore ability, so really there's nothing going for or against you here either.
Magus: I firmly believe that whoever wrote up the suli racial abilities was specifically thinking of a Magus when he or she did so, but then forgot about it and gave them -2 Intelligence. If they didn't have that Int penalty, you could make the best Magus in the game with a Suli. As it is, you can make a very good Magus, and the racial archetype, the Elemental Knight, helps with that. First of all, you get access to a few new Magus Arcana, including one that lets you expend rounds of Elemental Assault to add damage to elemental spells. You can also reflect spells that deal energy damage back at their casters. Finally, you can increase your energy resistances using arcane pool points. On top of those new Arcana, you get Incremental Elemental Assault as a bonus feat at 4th level, and can spend rounds of Elemental Assault to regain arcane pool points. Overall, this really allows a suli Magus to make the most of her Elemental Assault ability, and I strongly suggest taking the Extra Elemental Assault feat at least once to increase your rounds per day. The favored class option is also really good, giving you an extra arcane pool point every four levels, which you'll need since you start with fewer points thanks to that pesky Int penalty.
Monk: Wow. Suli monks can destroy face. Ok so you've got a Strength bonus to add to your flurry damage. You can use Elemental Assault to add elemental damage to your flurry of attacks starting at 1st level. And on top of all that, the favored class option adds to your elemental damage when using elemental assault! This eventually can add +10 to your damage on every single attack! You'll definitely want to use some feats on Incremental Elemental Assault and Extra Elemental Assault to get the absolute most you can out of this ability.
Oracle: Any race with a Charisma bonus is going to make an excellent Oracle, and the Strength bonus that goes along with it strongly suggests the Battle mystery, which can make your Oracle into a really great melee attacker in addition to the full spellcasting that you get. Obviously, you could also choose any of the elemental-themed mysteries, such as Flame and Waves, if you prefer.
Paladin [Antipaladin]: The bonuses to Strength and Charisma make for a very strong Paladin build, and I'm actually surprised that there isn't even a racial favored class option for suli Paladins. This is another build that can benefit strongly from Elemental Assault, but Paladins don't have as many feats to blow as Fighters do, so you might want to go with just Incremental Elemental Assault, but not Extra Elemental Assault for this one.
Ranger: A melee Ranger build will work very well for you thanks to the Strength bonus, but there's nothing else that makes this a better choice than Fighter or Paladin, really. Make sure to take Incremental Elemental Assault if you have a feat slot for it. The favored class option is decent, but you do have to pick which resistance you boost each level, as opposed to increasing all of them.
Rogue [Ninja]: Lack of a Dexterity bonus isn't great for a Rogue in general, though you can build a more Strength-based Rogue instead which can work very well. Ninja gets a bit of a boost thanks to the Charisma bonus.
Sorcerer: Sorcerer is a great choice for a Suli, especially if you plan to go into Dragon Disciple eventually, as the Strength and Charisma bonuses just beg for that prestige class. If you're not going Dragon Disciple, you'll obviously want to take a bloodline that makes good use of elemental-based spells, such as the elemental or orc bloodlines.
Summoner: Other than the bonus to Charisma, you don't have anything specifically going for you any more than a Sorcerer would, so this is a fine choice but not really optimal.
Witch: An Intelligence penalty means you can never be an optimal Witch. Go with Sorcerer or Oracle instead.
Wizard: As with the Witch, you'll never be an optimal Wizard as a suli. Be a Sorcerer instead.
Racial Favored Class Options:
Magus: More arcane pool points are ALWAYS helpful, so this is worth taking at every level for sure.
Monk: Monks can make even more use of Elemental Assault than most other characters, and this favored class option just accentuates that even more, adding up to +10 elemental damage to EVERY ATTACK by 20th level!
Ranger: This is a decent option, as elemental resistance can save your life, but it's situational since you have to pick which resistance to boost at each level.
Elemental Knight (Magus): This is an excellent archetype, if only you didn't have to suffer from the racial penalty to Intelligence. You get access to three new Magus Arcana, which let you make use of your Elemental Assault racial ability in interesting ways. First, you can add damage to your spells by expending a round of Elemental Assault, which is great because it stacks with the damage you're already adding to your weapons using the ability when you deliver the spell through your weapon. You also get to reflect elemental spells back at their casters at higher levels. Finally, you can increase all four of your racial energy resistances by 5 by using an arcane point. In addition to these new arcana, you also get Incremental Elemental Assault for free and can sacrifice rounds of Elemental Assault to regain arcane pool points, which can come in very handy!
Extra Elemental Assault: This does just what it sounds like... it gives you two more rounds of Elemental Assault per day. This is only worth taking if you're playing a melee character and you've already taken Incremental Elemental Assault so that you can spread your rounds per day out.
Incremental Elemental Assault: This is a seriously awesome ability, allowing you to spread your Elemental Assault rounds out instead of it just being once per day for X rounds, and anyone who wants to make use of Elemental Assault should definitely take this feat. You get it for free as an Elemental Knight, but a Fighter or Paladin should also seriously consider it.