Planetside 2. It's a decade old MMO now but still getting substantial updates. It can be quite frustrating because it has shooter gameplay with an effectively open lobby so the teams can be very stacked and you aren't given any matchmaking to make your first sessions easy. There's a lot of confusing and incoherent-by-design stuff with its massive combined-arms PVP scope. It doesn't have enough player population to be a really complete experience round-the-clock now. As well, there's the usual laundry list of old bugs and accumulated cruft that may never be addressed. But the deeper you go with it, the more evident it is that there is no equivalent game. Battlefield gives some of the flavor(and PS2's moment-to-moment is largely copied from BFBC2), but it's far more tame and controlled.