Whatever kit you choose, make sure it's a half orc if you intend to keep the kit. The half orc's easy to obtain 19 strength will help tremendously, especially on backstabs, and 19 is 5 numbers higher than an 18 strength.

As Armisael said it depends on the playing style. As I've mentioned previously I once did a game where I played four PC thieves that specialised in one or two areas.

Sawshbuckler - Pickpocketer

Assassin - Stealth

Bounty Hunter - Lay Traps/Detect Illusions

Dwarven Fighter/Thief - Lockpicking/Remove traps

I had a blast!

Pure thief class methinks.

Swashbuckler is good, because he get's more thief-skill points and is a better fighter because he can specialize in weapons. From role-playing aspect the assassin is the coolest, and since I really like the backstab (with poison!) ability, this is the kit for me.

If you are allowed to dual, then I think kensai/thief is the best.

Low Level - Swashbuckler
High Level - Assassin
overal - Assassin

ARTEMIS ENTRERI my hero...I like the assassins anyway I made enteri's dagger as an item for Bg2 basicly i modfied soultaker somewhat

Thieves are great because they rise up levels very quickly. This makes them ideal for dualing. Personally, I would go for a swashbuckler dualed to a mage at say level 10. You don't need to go higher than level 13, 660,000 EXP, to get a x5 backstab( not Swashbucklers obviously). This means you can still get to a very high level with another class. Somebody said that Assassins get a higher back-stab multiplier but it is not mentioned in the rules or game. Is this true and if so at what levles does it happen?

Au contraire, the fact the thief levels fast makes them one of the hardest to dual class, because you have to wait 10-11 levels to get those skills back, a very long time in the life of any other class. This makes them optimal for multiclass.

I'm not a big fan of thiefs dualed with mages myself, prefur to always have a fighter of some kind as one half of a dual classed character. although I did do a Swashbuckler/mage once at that wasnt bad atall, just prefur a pure thief or a fighter/thief, kensai/mage rocks though, if u havent alreay tried the former then I surgest u do so

I've wondered about this topic for quite the while as well.

Bounty Hunter Thief: Not very good, all he gets are traps which are very much luxuries, good term Accord.

True-Classed Thief: Not bad yet the bonus thief skills aren't all that impressive.

Swashbuckler Thief: Not half bad, somewhat better than the true-classed option yet you will eventually have to admit that a thief isn't the worlds greatest fighter, that's what your fighters are for.

Fighter/Thief Dual-Class: Somewhat ebtter than a swashbuckler but your still trying to mix a thief with the ability to fight, not such a good idea.

Assassin: Very cool, I think thier back stabbing ability is a must and they got talent. Style too. Would have to be the best if this didn't exist:

Gnomish Multi-Class Illusuionist/Thief (Like Jan Jansen): I was a bit wary when I thought about mixing a thief with a mage, I mean how useful could the mage spells I get be and how badly will my thief skills be reduced? Incredibly my Gnome is nearly as powerful as my true classed mage! He alone fulfills all of my thieving needs and it can still kick some royal ass when backstabbing! I'm still nearly starting out and he already has a 5x backstab multiplier causing him to do about 70 points of damage and in the pretty likely chance that he does a critical hit I do a whopping 140 points of damage! Instakill!