Happy 3 is too slow. Slower than even H2 IMO. I'm basing this entirely off of movement speed and map traversal. I like the speed of Halo 4 and Halo 5 the most.
FYI, Halo 2 and 3 have the same movement speed. Halo 3 feels slower because the smaller field of view makes it look that way. Halo 4 and 5 have sprint, but the maps have been scaled accordingly such that the traversal times remain about the same as in Halo 2 and 3.
If we're talking about "pace" or "speed" of gameplay we're talking about, we should be careful to specify what we're referring to, because there are many different metrics that aren't necessarily compatible (as in, one game may be faster than another in one, but slower in another): maximum movement speed, average time to encounter, average time to death. The first is the easiest to get data on, and the second roughly corresponds to map traversal times (and hence depends on average map size).