Along with Iron Man 2, the Fantastic 4 slot is my favorite game in Playtech's Marvel Series. In fact, if you bring in the Great Blue slot too, then you have my 3 favorite Playtech slots bar none. Arguably my 3 favorite online slots bar none in fact. Well, there are a couple of others but this isn't the time or the place!
The thing about the Fantastic 4 slots is the innovative free spin feature. I'll try and explain it: when you get 3 or more planet symbols in view (reminds me of Cryptologic's Silver Surfer game) you get 12 free spins (no retriggers). During the free spins, the center reel has stacked symbols and when one of the Fantastic 4 stacked symbols land, you get 4 extra spins with a special feature.
The extra special features are all different: Mr Fantastic adds expanding wilds to each reel, the Invisible Woman adds an increasing multiplier to each win, The Thing adds sticky wilds (but only gives 3 spins) while the Human Torch turns all the characters on all reels to Human Torches and makes the first reel wild.
The potential for big wins is very decent, especially during the feature game but it's really the Human Torch and The Thing features that provide the big kickers. You can get decent hits from the wild symbols in normal play and 5 of a kind hits too while the high-pay symbol is Mr Fantastic.
In my opinion, not only is Fantastic 4 more entertaining than feature-heavy games like The Mummy but also provides the potential for bigger wins. Right up there with the best slots online and easily Free Slots 24/7's most popular game.