I say the Knights were the most fun/challenging. They're as tough and as capable as elites, but they can transport to a different area, behind cover for example where the elites only have overshields and active camo.
As for pure fun, though, I say Hunters. There's a lot of satisfaction in getting that Magnum round in for the one shot kill, or for pumping their back full of buck shot.
Hunters for sure, every single time I see those bad brothers roll up I always know the fight's gonna be intense. But if I had to pick a back up it would be the lovely grunts, if only because of their hilarious quotes!
Hunters. They didn't show up very often so each encounter felt like a big thing. It is so fun dancing around them and trying to hit them in the back while you dodge their shots. Halo 5 perfected the Hunters, they are the most challenging and fun iteration in the series.