The Imperial Knight Houses, Chaos Knight Houses, and Titans for your stereotypical mechs.

One could argue that Tau Battlesuits, Space Marine Dreadnaughts, Chaos Dreadnaughts, Hellbrutes, Killa Kans, Deff Dreads, Wraith Lords, Paragon Warsuits, Mortifiers, and Penitent Engines are just smaller mechs.