My Top 5 Best (chronologically listed, of course):
1. Marvel Superhero: Contest of Champions (1982)
2. Marvel Super-Heroes: Secret Wars (1984)
3. Heroes Reborn: Heroes Return (1997)
4. Avengers Forever (1998)
5. Ultron Unlimited (1999)
Best:
Secret Wars (original)
Infinity Gauntlet
Operation: Galactic Storm
Annihilation
Inferno
Mutant Massacre
X-Tinction Agenda
Worst:
Civil War (both)
House of M
Secret Invasion
Age of Ultron
Avengers vs. X-Men
Infinity