When choosing among the available NPC's for party thief, who is the most viable and useful long term? I've often had trouble considering any of them very useful at all, aside from pre-IA Imoen.

For this discussion, I'm not considering situations where the protagonist is a thief, nor multiplayer (created) NPC thieves. I know that it's possible to make the 'perfect' thief this way. This is instead for players who want to use an existing NPC with scripts, banters and the like.

So among the original set of thieves, we have Yoshimo, Imoen, Nalia, and Jan.

Two of these can be written off right away:

1) Yoshimo - well, most of us know why we generally don't think of Yoshi as a "long term answer" to fill the role...(and if you don't, I'm not going to say anything more. Good luck.)
2) Imoen - no longer a thief with her change to Sorcerer in Improved Anvil.

This leaves us with Nalia and Jan...

Nalia is a Thief-4, with what I consider to be woefully inadequate ability in picking locks, even with the redistribution of her thieving skills. And you can just forget doing anything sneaky with her, she has no other skill at the "sneaksman trade" at all. I know that many people find her annoying, too, but she can rise to great heights as a mage later in the game.

Jan is a reasonably capable thief and an Illusionist, which makes him useful - if you can stand him. He's multiclassed, so progression in both classes is slow. He's a liability in combat for the most part, and I'd imagine he's not most people's idea of a good secondary mage-type to have around (but I might be surprised.)

It's my opinion that Nalia should at the LEAST be pumped up to be exactly what Imoen was before the IA change: Thief-7/Mage-8 (dualled). This would give Nalia 75 more points worth of thieving skills to apply to the big one - open locks - and to stalk/hide etc.

I'm sure players have other enterprising solutions among Mod-added NPC thieves and with multiplayer NPC-created thieves. I'd like to hear more on how you guys handle the situation.

If I use Nalia I simply use potions a lot.
Potion of invisibility -> Hide/Stealth alternative
Potion of Master Thievery -> Pick Locks, Pick Pocket bonuses
Ring of Danger Sense -> Detect Traps
You can also use Minor Spell Deflection to trigger certain traps without suffering from the consequences.

I've often also created my own thief character since I just like their gameplay better than mages and tanks.

Nalia can be decent enough as Iroumen says with the adequate boost ofcourse. I sometimes use a custommade Swasbuckler 10-> Cleric X for thiefing purposes. One of my first PC's back in the SoA only days was with this character, and it is awsome atleast in SoA, I never tried it in ToB. Such a character can easily solo the game but that's not even funny anymore.

If you really don't want to drop/modify a party member for a thief then don't bother - they're not essential by any means. All traps can be avoided or survived with a little preparation and Knock sorts out locks.