Infinity Gauntlet started off with an interesting premise but its simplistic plot ultimately failed to generate any real drama or excitement, and it fizzled out with the well-worn reset-button ending. That said I'll admit it had a few fun moments here and there. Historically though it is a pretty forgettable story with the single exception of it fleshing out the newly introduced Infinity Gauntlet. While not dire like many of Starlin's later efforts, looking back it probably did contain the seeds of his later mediocrity and marked his descent into recycling past stories over and over and turning his works into his increasingly fannish "my pet characters can beat everyone else's characters" stuff. Definitely not top 20.
COIE - well it may have been a bit clunky in parts, but there's no doubt it set the standard for massive universe-shaking mega crossovers. Both in the industry and in the comics themselves its influence is still felt. And in a few places it did manage to have some real drama and suspense, and some genuinely emotional death scenes (and not just for the big characters). It was far from perfect but I think holding its place in this year's top 10 after so many decades is justified.