I loved 1 & 2, then I kind of lost track of the series for a bit, but I heard everything through Halo 4 was pretty decent. But honestly, I don't see any reason for the story to continue. Basically:
- Halo: CE - introduction to the Covenant and the Flood
- Halo 2 - learn about inner workings of the Covenant and history of the Flood
- Halo 3 - Covenant brings the fight to Earth
- Halo: Reach - prequel to Halo CE - adds backstory about human intersteller colonization
- Halo 4 - looks like a kind of side story continuation of Halo 3, with new enemies
- Halo 5 - Cortana turns evil?
I haven't played anything past 2, so maybe it hits different in the games, but the series really looks like it loses its purpose around Halo: Reach and Halo 4 and becomes a cash grab. They really should have wrapped things up around there, at least as far as Master Chief is concerned, and build a new series in the same universe (or a completely new universe).
And that's also why I don't bother with Assassin's Creed games anymore, they've long since lost the plot, and doing the same thing but in a different setting just isn't interesting enough for me.