I remember being blown away when i read the Infinity Gauntlet, and I really liked the Galactus vs Inbetweener battle. Honorable mentions to Galactus showing the Sphinx just how far ahead of him he was.
All three great battles stoic,that one with Galactus and the Sphinx.Made Galactus my favorite character of all time.The bad thing about it is ,he is also the most popular cosmic of all time(#5 in wizards all time villen rankings).So if you have a new cosmic come in and want to make him bad asses,have him beat Galactus and your in like flint!thats why G was a jobber until they reduced his stature so much,they had to make him look powerful again!
I haven't enjoyed many high-order cosmic battles, but I thought the Thanos Imperative stuff was cool. Both when the Cancerverse invaded and when Death nuked their faces off.
I'd mention the Annihilation stuff, but I think that was more just the entire event being awesome.