BW are very very good at what they do, and have made several games that quite genuinely belong in the canon of "greatest in the genre", such as BG2, ME, KOToR, and DAO.

They've also made some games widely regarded as mediocre (Jade Empire) or flawed in some significant manner (DA2), even though I personally like those a great deal. Overall though, they're an extremely good dev company and their hit rate is fairly impressive.

Bethesda on the other hand made Morrowind, which means that unless you're pitting them against either Firaxis (For making SMAC) or Ion Storm (for Deus Ex), Bethesda wins the "Best developer" contest without question. Always. Against anyone. Not even Oblivion and Fallout 3 are enough to change that.