A large number of the initial batch of slots uploaded to Free Slots 24/7 are developed by one of the biggest online casino game software developers, Playtech. I've been playing their slots games online since around 2008 when they first launched their impressive Gladiator game and to be honest, until that point I hadn't really rated their slots very highly.
Prior to 2008, the best offerings from Playtech were 'A Night Out' and 'Desert Treasure' in my opinion but compare those graphically to the latest releases from playtech, games like the lovely-looking Kong, the aforementioned Gladiator of course and some of the Marvel slots and you'll see a marked difference. The 2008 release of Gladiator raised the bar - and not just for Playtech but all online slots developers.
The main problem with Playtech though is that their licensing regime isn't that tight. To be fair, the majority of Playtech casino operators these days are what you would call 'safe' - for example, all those that are listed on this site I have played at myself and I've never not had one pay me. The biggest issue is that some can drag their heels on payouts and some even have cashout limits. But not the ones you see here at freeslots247.net - I wouldn't tolerate that personally.
It was really the introduction of the Marvel slots that made people sit up and take notice of Playtech slots and you'll find most of those here. My favorites are the Fantastic 4 or Iron Man 2 but the latter is pretty volatile so watch out! The others look good but whether you like them will depend on the game-play in all reality - some are better than others. The downside is that in 2016, Disney are withdrawing the Marvel gambling licences (brand protection) so these games will be replaced but with what, we don't yet know.
Another couple of games outside of the Marvel series I'd point you at are the cult classic Great Blue which has high multipliers in the free spin feature and stacked wilds and also the nice follow-up to the original Desert Treasure slot, imaginatively titled Desert Treasure 2 which is slightly more volatile but has expanding wilds on the center 3 reels.
Overall though, Playtech slots are right up there with the best online games IMO. As long as you choose a decent Playtech casino you can have lots of (sometimes expensive!) fun on slots with RTP's in the 90-95% range according to their manual. Note that any slot with a progressive jackpot will generally have a slightly lower RTP.