I'd place it fifth, just above Halo 4.
So...
1) Halo: CE
2) Halo 2
3) Halo 3
4) Halo Reach
5) Halo 5
6) Halo 4
While Halo 5 has a campaign that's arguably worse than 4, it's saved by superior multiplayer. Arena doesn't approach the greatness of 3, but it's definitely better than the mess in 4. Warzone is a really nice addition too.
The series still has a long way to go to reach (no pun intended) Bungie's glory years, but Halo 5 is a baby step in the right direction.