Out of the games I've actually played the worst would ahve to be Rifts or WEG D6. Rifts is too clunky, too unbalanced, too everything. The idea of the setting is cool and the setting itself is very good in places, but in others it's just boring. D6 system or "Buckets o' Dice" wasn't too terrible at lower power levels, but when you have to start rolling 6-10 d6's minimum, it just gets out of hand fast.
Rolemaster ranks right up there as well.
I've seen Synnibar and FATAL as well as a few others, but I never played them. Those looked FAR worse that anything else.
Kane