I always assumed BioWare specifically chose Obsidian to hand off the creations of sequels of NWN and KoTOR to because they were closely related developers, both having worked on Baldur's Gate together (then they were Black Isle). They also made some pretty good Infinity Engine games after BioWare left it to move onto NWN and such

Aside from Obsidian's frequent name change (though I think it was actually legitimately closing and reviving simply with many of the same people; I think Troika was in there somewhere as well, and if memory serves me inExile is part of that pedigree as well)

But as far as actual BioWare games, the Baldur's Gate games are grand, but almost too grand. I haven't completed any of them because they're so vast. Though I thoroughly enjoy the 20-30 hours I plug into them each time I go back, I can't pull myself all the way through

NWN did nothing for me. I felt obligated to try to get through at least one of the main story campaigns, but the first one was so horrible, I probably didn't even put 5 hours into it. NWN 2 is so much better for a single player game. Score one for Obsidian there

I loved both KoTOR games pretty equally for different reasons, so Obsidian did a really good follow-up there, because they're both stellar. And for the longest time KotOR was one of my all-time favorite games. I still hold it in high regard, but I think Mass Effect successfully replaced it. But its still leagues better than the prequel trilogy of movies for a Star Wars experience

And even though I've been a BioWare fan for the ages, I'd almost have to agree Mass Effect 2 was something special. So fast, and frenetic, with well told, dramatic and intense story sequences, lovable characters are introduced and reprised, and the combat is so crazy and fun, it's hard to compete with that kind of experience

I appreciate Dragon Age, and am happy to own them all, but its just too paint-by-numbers hardcore fantasy fair. There's not enough good Sci-Fi rpg goodness in the world. And I'll always be happy to go back to Mass Effect. Especially #2