While I'm in a nostalgic mood, here are a couple of classic Fantastic Four covers that I remember fondly from my dim and distant youth. At left is the first issue of FF I remember owning, #22 (cover date January 1964), in which the Mole Man returned from issue 1 and menaced the Four with a deathtrap in his mountain stronghold. Each member was separated into a chamber designed to neutralize their powers: Reed Richards, for example, was trapped in a room which filled with cement which restricted his stretching ability. The next cover, #35 (May '65), featured the story "Calamity On The Campus" and depicted the FF's struggle with the alchemist Diablo and his creation, Dragon Man, a huge statue which Diablo brought to life with his potions. The Four were visiting the University of which Ben and Reed were distinguished alumni in order to give a speech, Diablo did his thing, and all hell broke loose in that outstanding Lee/Kirby fashion. This story had action, humor, even a little romance (if I recall correctly, it was in this issue that Reed proposed to Sue Storm, the Invisible Girl) and it entertained five-year-old me so much that I still consider it one of my favorite comics to this day.
If you're interested in Marvel's Silver Age (roughly 1963-1970), then you should check out this wonderful site, the Silver Age Marvel Comics Index, which features cover scans from almost every classic Marvel comic of that period, and includes reviews on several. Hours of fun and nostalgia may be had by visiting, plus the opportunity to see those cool covers.