I have a different set of favorite 'mechs depending on which game.
Mechcommander - Hollander II, Nova Cat.
Mechwarrior 4 - Hunchback with Heavy Gauss and
Mechcommander 2 - Jagermech, Hunchback and Hollander.
Mechwarrior Online - Uller/Kit Fox-Purifier Hunchback-4G, and Blackjack-Arrow
Over all probably the Hunchback.
Uller looks coolest IMO but it's not particularly good in any Battletech game as far as I'm aware.