Mostly Post-Osiris Units. There are some Pre-Osiris Units that can be considered meta like Inoko, Serin, QT, Sennie, Peter, and such.
When it comes to harder contents of the game, you'd need to have a wide pool of Omni units for the purpose of strategies. There are still Pre-Osiris Units that are truly capable of being in hard mode contents but the majority of the meta right now are Post-Osiris units. That doesn't mean all Post-Osiris units are really really good though, some exceptions are judged by others here.