Honestly all of them are very good (talking about the street cars...also not sure about the 120i, never tried it).
In fact, I can't think of one of the FR street cars from BMW that would be a "bad" drift car. I'd actually say most if not all of them are above average.
Grab all the BMW street cars in the OCD right now and you'll have a handful of some of the best drift cars in the game.
To answer your question directly, I'm not sure it's really possible to say one of them is the best, as they are all just that good. But, if I had to put one at the top of the list or get kicked in the junk, I'd say the M3 GTR is the best. It Is blessed with perfect weight distribution, is stupid fast, and handles like a dream...a combo which lands it on the banned cars list in most comps...but it's still hella fun to drift 👍