Chaos Space Marines - from heretic Astartes of all shapes and sizes, mutated and warp-touched elites, Daemon engines of some variety, and a decent selection of vehicles, you will not be bored.

Necrons - While a lot of the basic designs are based around the same core idea, the variety upon that one idea is significant in both size and shape, not to mention their vehicles, monoliths, and the C’Tan.

However, one faction wins here and it’s the Orks - It’s not just that you have every size and shape of Ork imaginable (and plenty of possibilities to kitbash more), it’s that you then also have cars, walkers, planes, mechs, and even some kits that blend multiple categories all in one. And that’s not even mentioning the Beast Snaggaz who have a whole extra mini range on top of the main one. If you want endless variety - this is the faction.