Bioware wins this, regrettably.
Compare.
Let's first look at the 3 RPGs developed by Bethesda in the past 10 (!) years: Morrowind, Oblivion and Fallout 3.
1. Morrowind has nice mushroom towers and creative stuff, but it plays like shit.
2. Oblivion is shit across the board. story, dialogue, game mechanics, art, level design are all bad. Beth even made Sir Patrick Stewart look bad.
3. Fallout 3 then. Great setting; poor shooter; simplistic RPG; shitty story and dialogues. Too bad Beth didn't invent that setting; they just bought it from Interplay.
Mediocre as Bioware are, their stories, writings, voice overs, visual designs and animations are still much superior to Bethesda's stuff. Not even a fanboy can deny this. So on a purely technical level Bethesda games cannot hold a candle to Bioware.
Gameplaywise then Mass Effect is a much better shooter than Fallout3, even with the mediocre gunplay in ME1. So Bioware do the actual game better too.
About Square Enix we can be short: they gave JRPGs a bad name with the pathetic FF series and their Disney crossovers.
They do publish some great games, made by other developers, but their own stuff is the worst crap.
So Bioware is king in the land of the blind.