Well, this modifies the question a little. It's hard to compare older sets (pre Masques) to newer sets. I would have to automaticaly omit any of the basic sets (post Unlimited) just because they have too many cards from the expansions and quite a few of the "stand-alone" expansions (Ice Age, Mirage, Tempest, etc) depended on basic cards to get by (Dark Ritual, Counterspell, Incinerate), so would that remove them as well?
Still, I think that Ice Age was the worst of the older sets. From the newer sets I didn't like Legions. Sure, it had some great creatures (all creatures, as a matter of fact!), but it took so much of the creativity from the game by practicaly building the decks for you...
-Ferret
"Check out my TOTALLY ORIGINAL Elf deck!"