The Monkees - Peter (yet he's my favorite)
The Who - John (but only because the other three were absolute monsters)
CSNY - Bing (just kidding)
The Band - The wrong answer here could bring down the Steve Hoffman forum.
Fleetwood Mac (Tusk era) - Oof, tough one, its a tie between the two dudes the band is named after. YET they deserve gold medals for being humble enough to give Buckingham/Nicks the freedom (and the drugs) to do their thing. Anyone who says Christine better back it up with FACTS.
Original Stones - The magic is that the weakest link always changed from album to album...Starting out, I might even say Keith was the weakest link, but then it became Brian, then Bill...and so on.
The Cars - Easy to say Greg Hawkes, but take him out of the mix and these guys are just another power pop band from Boston. Plus, he gave short, geeky guys hope of being rock stars themselves one day.
R.E.M. (original four) - Every possible answer will get you shit.
Ramones (original four) - Tommy (but he was producing, so he gets a pass)
Ramones V2.0 w/ Marky - Dee Dee most of the time.
Smashing Pumpkins (original four) - Three-way tie, but perfect for the situation. We singer-songwriters who never found our "forever accomplices" should all be so lucky.
Kiss (original four) - Peter, but, then again, no REAL drummer would have gone along with the make-up thing at the time... AND, even though the band has had "better" drummers since, their feel on those Criss-era cuts has always been so heavy-handed that you don't mind Criss's playing so much. Even so, one dreams of what Destroyer/Love Gun (the best era) would have sounded like with a dude who swings like Steven Adler.
Which brings us to
GNR (appetite line-up) - WOW...think I hear my mom calling...
Aerosmith (classic line-up) - probably whichever guy you can't name off the top of your head...in my case, bassist Tom Hamilton.
Point being, as demonstrated by MOST of the above musicians, the weakest link in a great band is usually still a bad-ass.