Great Blue is the slot for me. It was the first game I added to the Free Slots 24/7 inventory because it's my favorite game. But a word of warning: it's become a bit of a cult game this one but it is extremely volatile. Only Playtech's Iron Man 2 slot featured on the site comes near Great Blue in terms of volatility so if you ever lay this for real, treat it with respect.
The reason the Great Blue slots have become a bit of a cult is because of the high multiplier potential in the free spins, coupled with the fact it has stacked wilds on all reels. It is possible to get a 15x multiplier in the free games (2x, 7x, 10x or 15x) and you'll get between 8 and 33 free spins. The higher the multiplier then generally the fewer the spins but the minimum you'll get is 8 and thats' rare. It's normally 15 - 23 with the 7x or 10 multiplier and 25 - 33 if you only get the 2x multiplier. Plus they can retrigger.
I could show off some lovely screen shots from this game. I have had 500x on several occasions from getting a decent multiplier in the free games and a reel or two of stacked wilds. I've even had a 500x in normal play on occassion but you need 3 or 4 stacked reels for that.
Great Blue is just a great game if you have the patience to wait for the free spins. If you don't, then avoid it and play something like Blade which has an increasing multiplier in the free spin feature up to 6x.