I don't like to pick on people in general, but here goes ...
Vocals: Alanis Morissette. Good God, who gave this woman a recording contract? She has the most nasally, whiny, strangulated voice I think I've ever had the displeasure of hearing.
Guitar: It pains me to say this as a fan of Yes, but I'm going with Steve Howe. He's always been good on acoustic guitar, but even in his heyday in the '70s, his playing was brittle and rough on electric guitar. It always sounds like it's a monumental struggle for him to move his fingers up and down the fretboard, as if he's scared to death that he's going to flub a note. And any electric guitar work he's done in the past 10 years is, quite honestly, embarrassing to listen to.
Bass: Roger Waters. Again, I'm a huge Pink Floyd fan, and I respect Roger's artistic merits and his creativity. But look back at his playing even in the early days. Very tentative, and usually built around one or two simple notes. It's never been widely reported, but Gilmour actually played a lot of the bass parts on the classic Floyd albums, simply because he could play the parts better than Waters could.
Drums: Carl Palmer. The guy was fast and flashy in his ELP days, but he absolutely cannot keep a steady beat. On the old ELP albums, you can hear how much he rushes the beat. Even on Asia's "Heat of the Moment," a straightforward 4/4 rocker. The end of the song is about twice as fast as the beginning.