What early access game that you know of, looks the most promising?

killing floor 2

Squad - filled to the brim with potential. If they manage to improve performance and add vehicles, I can see it becoming a huge favorite of mine.

7 Days To Die - the last big update they did really changed and improved the game a lot. Still has a long way to go but this might turn out to be one of the best survival/building games yet.

Pulsar:Lost colony
http://store.steampowered.com/app/252870/
If you are a star trek fan you should definitely check this one out! I have sunk a good chunk of hours into it already and they update pretty often with a UI overhaul coming in the next few days!

7 Days to Die, because I already have over 200 hours in it.
Layers of Fear looks promising, I'm looking forward to playing it once it's finished.

Secrets of Grindea looks promising

Also 20XX

ALSO I found Forsaken Isle today thanks to the discovery queue.

Well, most games are early acces xD but the most interesting ones are:
-7 days to die :D
-Stranded deep
-The forest
-Besiege

and another game that has some potential is Down to One

Terra Incognita has not been mentioned too. Seems to have potential.

Squad - Project Reality, but in Unreal Engine 4 instead of Refractor Engine 2 (Battlefield 2 engine)? Yes, please!

Killing Floor 2 - I really shouldn't have to explain this one.

Interstellar Marines - Well...it's been in development hell for at least six years, with initial work dating back all the way to 2005. Not quite Duke Nukem Forever's fifteen years of development time, but depending on how you count how long Interstellar Marines has been in development, it already beats out Team Fortress 2. That said, they have a pretty solid foundation, and I'm looking forward to seeing if the devs can follow through with the rest of the game.

Angels Fall First - What the new Battlefront game should have been. Probably. Still incredibly early in development, with many of the bugs, UI and control issues you'd expect from such. What's there is promising, though.

Haven't played the following games:

Project Zomboid - Looks to be the single best zombie survival game. Well, Cataclysm: Dark Days Ahead might be better, but that depends on how willing you are to deal with text-based graphics. That said, haven't play either of them.

Layers of Fear - What I've heard is fantastic, but I haven't looked too far into it because I enjoy surprises.

Distance - Nitronic Rush was pretty good, and this looks good, too.

Grim Dawn. :)

Dead Realm: http://store.steampowered.com/app/352460
Grim Dawn: http://store.steampowered.com/app/219990
Savage Lands: http://store.steampowered.com/app/307880
Who's Your Daddy: http://store.steampowered.com/app/427730

Crawl and space engineers
They both already great, crawl would be hit in its genre, SE could be classic game, if devs would make everything they promised, well even 80% of it, they got long time ahead, but they are making it perfect ea project with great updates every weak

Grim Dawn, which is about to come out of Early Access soon :D

Phantasmal

I want to say Interstellar Marines but have been over a year since I played it and not sure if the development has been good or bad.

dig or die, looks promising :)

That would be The Forest. Its an awesome game and thats still in Alpha!

Crosscode. It was in a Bundle Stars bundle a while back but I can vouch that it's worth the full price.