I was looking into making a Soulknife Rogue character and, as always, I am stumped on what race to choose. Flavorwise I was thinking that a Gith would make a good fit, seeing as Giths are psionic, and they get some good all- around features. But I wanted to know what other options are available. In particular, I'm looking for something with good DEX and surviveability.

I played as an aasimar and channeled my inner divine wrath to knife people with my brain. But I don't really care about having a race's ability score match the optimal build for my class. 

Any race with a Dex bonus is mechanically a good option, and survivability will depend much more on non-race factors. Is there anything you're interested in beyond being a mechanically effective Soulknife? Rogues get 4 skills, background likely adds two more and race might even increase that further, so you could be a pretty good investigator or diplomat or performer or be knowledgeable about a host of subjects even with a mediocre bonus from the relevant stat for instance. Look a bit further than just class abilities.

I agree with Pangurjan, it isn't very hard to fit the Soulknife when it comes to optimization. You just need good dex and if custom origins is available then any species can suffice.

I like Dhampir for this quite a bit though. Extra movement means more with cunning action involved and always on spider climb starting at level 3 is amazing for maneuverability in and out of combat. Soulknives make good ranged rogues so climbing the walls and throwing some psychic blades from relative safety is a good strategy. It's fun to combine with Psyhic Teleportation as well. If you can reach a ceiling or other high surface with a knife throw then you can teleport there and just stick to it no matter if there is a place to stand.

Being flight-capable is always good - winged tieflings, aarakocra, and if UA, owlkin and fairies can all make relatively good soulknives.  Flight will make it easier to line up shots that don't have to overcome cover.

What I meant about surviveability was a race that offered something like a constitution increase or a race that offers extra hitpoints or something along those lines that helps you last longer in combat. I'm considering doing either half orc or yuan-ti pureblood and making use of the Customize Your Origin feature. Someone else also brought up that dhampir would be a good choice too.

That sounds pretty good! Both the dhampir and the reborn lineages offer some good oomph for this class. And if you consider the Ancestral Legacy feature, you could bring in a race with a flying speed so that when you can't reach that ceiling using teleportation and spider climb you can just fly there. I think you've given some pretty solid advice, HeironymusZot. Thanks.

Hill dwarf gives you an extra hit point per level, on top of a con bonus. Nothing for dex, though.

But if you're already using custom origins, can you move ability scores around (Tasha's rules)? If so, then that really opens up options.

If you want movement, don't rule out the Tabaxi as they are kings of movement.  What I find confusing in some of the answers is the ranged aspect.  A soulknife is good for ranged, up to 30'.  So you really want some mobility options as sniper is not part of what you do.  For me, this is nearly pure edgelord territory (almost as much as my warlock).  So an elf with a bit (aka  a LOT) of attitude fits the bill really well (or half).  And it's more than just always having a weapon... it's the confidence walking into a medieval tavern without a visible weapon or much armor and knowing you can take out the room if you need to.... but that you are built to do more dangerous things.  Feats are going to be important as you progress, especially those that work with your movement.  In the end, though, I think the attitude of a soulknife (and the dark secrets behind your gift) is way more important than the race.

Yuan-Ti Pureblood is great for their Magic Resistance and they also come with some solid innate spellcasting. The Poison Spray cantrip isn't gonna be your go-to at any point, but it's nice to have a Save-Based attack at the ready in case you're fighting something with particularly high AC (although it's a CON save, and most creatures with high AC also have decent CON).

Shifter is another Race with solid survivability boost, and it's very customizable, since you can choose what form your shifting takes. All the different forms give you temp HP, but Beasthide gives you even more temp HP and an AC boost.

If you really want to lean in to being the stealthiest Rogue you can, Mark of Shadow Elf is great.. you get a +1d4 to every stealth check you make, and by level 3 you get a once-per-long-rest cast of Invisibility.

I plan a reborn soulknifeto stack those hability check dices and see how high can i get. (even considering multiclassing to pact of talisman warlock).

With the tashas cauldron rules the dex modifier can now come for any race, in fact all the proficiencies and skills can, so my suggestion is check your DM is happy using the tashas rules and then focus on background and story knowing mechanically you have the freedom to tweak those bonuses. 

I made a Shifter Rogue and it looked pretty solid as TransmorpherDDS mentioned. I doubt I will ever play it.

Something I have to ask the OP and everyone else is; Do your groups use any race together in the same party? I would find this fairly relevant to make racial suggestions.

When reading through the races, some list racial alliances and many list reasons they wouldn't commonly ally themselves with certain races(reclusive, tribal, goblinoid ally, etc). It may be old fashioned thinking but, some of us players don't see all the races getting along. I'm thinking a sort of Horde vs Alliance split of races to put it simply and of course this split would have outliers.

Do most people just hand wave all this? 

In my games I leave it up to the players to work out how there characters feel about each other but with the guidance that there has to be a reason they are staying together long term. I also remind my players that in my world every intelligent creature that is not just an animal/monster can be any alignment, so goblins and orc settlements can be good. 

So while there might be friction between characters because of the race they are the characters know that the individual might be good. It very much depends on the setting, the DM and how the players want to roleplay. I have been both a player and a DM for parties who at the start where forced together for some reason and really didn’t get on at all, then over time friendships between characters where forged, trust was built and eventually the group became a fully formed party by level 7-8. Roleplaying those moments of friction can be fun, but players need to realise that for the party to stay together and not fracture you have to find some common ground within the characters. 

I love dhampir for this, one of their hungers choice is “phsycic energy” I imagine just creating knives and eating them😂

Call it a stereotype; but Kenku make great rogues.

i try to make my fellow characters feel as insecure as possible when around me. especially as a reborn lineage who watches them sleep

Im trying to make a soul-knife be a literal soul so anyone have a good one for that? bonus points if its very creepy

Quote from Th3_K1ng518 >>

Im trying to make a soul-knife be a literal soul so anyone have a good one for that? bonus points if its very creepy

Check out the Plasmoid species from the Spelljammer: Astral Adventurer's Guide. You are a translucent, amorphous ooze and can shape yourself into a shape resembling a ghost or spirit (or almost anything - there are limits to the number of appendages, heads, etc.).