I don't like the massive assault 'Mechs. In some situations, I favor the large ones, but not usually. Their just big and clunky to me. I like the smaller 'Mechs for the most part, mainly because of speed and agility, with a bit of firepower. If I can shoot, get behind a hill, and come around from another side, I'm perfectly happy. With the huge ones like Atlases, I don't find it entertaining just going in, all guns blazing, then flat out annihilating the opposition. In some cases it's fun though.
My favorites were the Cougar, Shadowcat, Uziel, and Vulture. Fast, and they can pack a punch.