With more than 14,695 video slots titles, it's easy to argue that slot has more players than any other online casino games. But "with great power, comes great responsibility" ~ Uncle Ben, this has become more problematic to prove that every bet made was fair and will be fair, that slot providers didn't cheat slot players.
Traditionally, players have to rely on trust, that every game has been through various testing to make sure its fairness. RTP, volatility, exposure, also checked at this phase. However, trusting this big entity is just like trusting the big banks, something that doesn't get along with the spirit of decentralization and "don't trust, verify" thing.
Hence, this thread will list provably fair slots only, so users can determine if the provably fair status can encourage them to play (or not). Anyways, feel free to post your favorite provably fair slots here, in case I forgot to mention it.
#1 Bitvest Slot
Legend says it is one of the oldest provably fair slots. The unique thing is, you can customize the pay table according to your own preference.
#2 BitcoinVideoCasino Slot
Not sure if this is one of the oldest, but it sure looks old (lol). The good thing is you can customize your pay line according to your own preference.
#3 Samurai & Egyptian Slots - Stake

Did you get an email to try Samurai Slots? I tried, and it's good compared to the "previous generation" of provably fair slots. It has scatter, bonus rounds, sounds, and unique reels setup, like Deadwood/Tombstone.
#4 Mintdice Slot
This slot has nice graphics, sounds, and has decent gameplay. The downside is that no scatter (and bonus round). Probably this slot is the most underrated one.
#5 (Some of) BGaming Slots

Probably this is the most developed provably fair slot game in terms of gaming experience. The downsides are: high house-edge (3-6%) and the data (hashes) are not recorded in users' history. Users have to check the fairness immediately before the next spin, which is pretty much useless. BGaming has at least 33 video slots title, and some of it already fairly popular, e.g., Fire Lightning, Platinum Lightning, Avalon, etc.
Sadly, no "verify" option founds on new titles like Elvis Frog & Book of Cats.
Feel free to leave comments here!

Disclaimer:
Numbers on this list don't represent rank or quality or anything in a particular order, it's just a simple numbering. Gambling is for adults who understand the risks involved with this game. Plus, OP wouldn't take responsibility whatsoever if you then become slots addict and lose lots of money after reading this thread.