It's all been downhill since BG2, so you should just play BG2 and call it a day. the Enhanced Editions are a good jumping into point, as they have been reasonably polished and ready to go on modern systems. No DLC nessecary for the EE's.
If you really don't want to do BG, then the 2nd best is KOTOR, as others have mentioned. KOTOR works, and then there's a great sequel afterwards (not Bioware, but builds on the themes in the first one).
I would personally avoid Neverwinter Nights -- unless you want to play the 2nd expansion pack: Hordes of the Underdark. Vanilla Neverwinter had a pretty terrible story. Hordes, on the other hand, still didn't have the greatest story but had a lot of really fun encounters, and kinda got better the further you went down.
I also don't mind Jade Empire, although it's short and the combat is...really hard to play nowadays. Still, kind of fun and kind of a unique game in their wheelhouse.