Marvel’s Top 70 Covers
To celebrate its 70th anniversary, Marvel is asking readers to cast their votes for the top 70 Marvel Comics covers of all time. You only have to choose five, but they've got a search tool on site that lets you find nearly any cover from any Marvel comic since 1939, which has a good chance of eating up the rest of your day anyway.
I say "nearly any cover" because an attempt to revisit the early 90s by searching "X-Men 2099" turned up nothing. This cover is also ineligible, although it remains the winner in my heart.