Tested pretty much every active private server over the years, Krypton is the only one worth playing imo.
runs in browser (PC, Android & iOS) or you can download the client which works better, I'd recommend that.
No LF4, no drone formations, 8 Iris. If you played back in 2011 you know exactly what that means. None of that "start with full lf3 and max drones" nonsense that makes the whole thing pointless after 2 days.
The dev actually gives a shit – updates drop pretty regularly and the server isn't one of those abandoned projects running on fumes.
There are 8-man gates that feel like actual dungeons with decent rewards, which gives you something to work towards once you're geared enough.
If you're just starting, join MMO company, they have the most active players. Ask around for Beta clan once you're in.
There's also a starter code if you want a small leg up on progression:
KR-1617-PSMV
Milestone Reward
Register 1k Uri + 3k MCB-50 + 25k Credits
Level 5 2.5k Uri + 50k Credits + 1.5k UCB
Level 10 5k Uri + 150k Credits + 300 RSB + Tokens
Level 15 11.5k Uri + 300k Credits + 7d Premium
Not game-breaking but it helps early on.