My top 3:
#1 - ASM 31 - Just gorgeous! Love the red primary and Ditko's Spidey head renederd in the center (I own this copy in near mint condition, one of the prizes of my collection - actually the other 2 listed I own NM to NM/M copies of as well)
#2 - ASM 135 - Classic Romita! Gives all the elements of the story in one shot!
#3 - ASM 300 - not a huge fan of the black suit, not a big fan of that era in comics but LOVED Michelinie's writing and McFarlane's art and this cover is classic Toddy.