I came into the series during Halo 2's reign, so whatever that is. I really, really enjoyed Halo 3's lifespan, as that was the game I sunk the most time into (it was literally an obsession) and all my friends were playing it too.
Huge fan of Halo 3: ODST's story. Liked Reach well enough. It was almost as enjoyable as 3 (multiplayer wise). Halo 4 was just OK, although I was actually better at this game than any other Halos. Halo 5 was kinda bad, apart from Warzone (which is great IMO). Its story is complete garbage.