The only NPCs in the original who come close to Jan's level of humor are Yoshimo and Imoen. Conveniently, you can take both. The NPC with the most interaction and banter (as measured by character count in associated DLG files) is Jaheira.
Without tooting our own horns too much, Solaufein and Kelsey (Mod NPCs) both have Jaheira-levels (or more) of conversation, banter and interjections. In addition, they both combo well (= funny, in the opinion of many) with Jan and Imoen (in particular, Kelsey's romance with Imoen is cute and funny). Since you're playing the game for the Xth time, now is the time to grab some mods (at least the ease of use mod, to make replaying less boring:
http://www.cs.berkeley.edu/~weimer/bgate ).
NPCs to avoid if you want party interaction:
Cernd
Valen
Some NPCs also form little "teams" that have a bunch of banter: Mazzy/Valygar/Korgan is fun, for example. Good luck.