Late to the poll-party on this one. Tough choice - have so many hours on the BG series, NWN series and KotOR series ... but if I had to choose just ONE, it would be BG2, so that franchise wins
I probably spend around 10k hours working on NwN modules, scripts, etc..
There will probably never be another game like it in regards to flexibility and ease of creation. You could literally design your own world.
Mass Effect as a franchise. All three games are solid with only the endgame (I'm not talking ending per se) of ME3 being a letdown.
The Dragon Age franchise suffer from uneven quality.
Jade Empire was interesting but only one game.
Most other franchises were only partially developed by Bioware. My favourite Kotor game was developed by Obsidian.
Baldur's Gate II is their best game by far though.