For me DDOT in Detroit takes the cake for the worst for obvious reasons. Waited an hour for a Michigan Avenue bus to come when trying to get from where we were staying in Southwest, to go to Corktown to drink. Most of the band I was travelling with ended up taking a Cab while the Drummer and I stuck it out on DDOT, out of curiousity.
I also am not a fan of OC Transpo in Ottawa. The "O-Train" was pretty poorly planned.
In terms of best I will show my hometown bias and give it to NYCTA, as much as it aggrivates me sometimes. 24 Hour subway is unbeatable.
Im also a fan of Vancouver's TransLink system. I love the SkyTrain.