I was born in 1990, so McCall is definitely the best manager we've had in my time. That's mostly down to everyone else not being very good though.
Dalziel had some good times but should have achieved a lot more, Reid wasn't too bad but couldn't keep us up in two attempts. Campbell Money, Shanks, Connor, Watt and Roberts were all rotten and I don't remember Stainrod and Burley.
As for bad signings, Graeme Brown cost us a substantial amount of money at a time when we were struggling both financially and on the pitch. I think he only scored one competitive goal for us (on his debut IIRC) despite being a first team regular before getting a bad injury that meant he never played for us again.
Also, I heard that we paid a large signing on fee to Thomas Gill before he buggered off after 6 games or something.