Awesome: It's supposed to be a PPC/laser boat, but it suffers from severe scaling issues. It just doesn't have the armor for its wide body. The hardpoints are also low, making you expose your whole upper torso when you shoot. Mostly obsoleted by the Battlemaster and Banshee. Still, it has good mobility for an Assault. 8R and the 9M are the better ones, 9M being a crazy fast energy/streak platform and 8R being the scariest LRM boat in this game(the only
Victor: The Assault that feels like a medium. Their mobility was nerfed for exactly this reason not long before, but I hear they're still very responsive with bigger engines. They have great hitboxes and big arms, capable of sucking an absurd amount of damage before going down. They're good brawlers, mediocre snipers and good "poptarts". It's a great choice if you're looking for your first Assault, a very forgiving mech.
Stalker: The original boaters of MWO. Whenever the meta changed, the Stalkers simply adapted and kept their popularity. Their sheer number of missile and energy hardpoints allow them to have great firepower. Their hitboxes are unique, frontally almost impenetrable but vulnerable from sides. Even from sides, getting to CT requires you to "peel" them and their CT still takes reduced damage because a damaged ST only transfers 50% of the damage received to the next component. Their weapons are almost exclusively high mounted, allowing them to dish out damage without any retaliation. Stalkers were nerfed indirectly by "Ghost heat" implementation punishing them for boating large amounts of PPC's/LL's. The hero Misery rose to the spotlight at this time and regarded as "the best mech in the game" for a long while for this reason, because it can pair 2xPPC's and an AC20, circumventing the ghost heat in an extremely tough chassis.
Though, Stalkers are very cumbersome and terrible at brawling ranges. They're best lurking behind the frontlines, abusing their high hardpoints and heavy energy weaponry or missiles. Some(including myself) find this type of gameplay extremely boring, but it's a lifestyle for some.
Battlemaster: Battlemaster is a mix of personalities, it can be thought as an upgraded Awesome or a more mobile but vulnerable Stalker in MWO terms. They retain high mounted energy and missile weaponry, but also gain arm mounted energy, missile and ballistics weaponry. They can mount the biggest engines and energy-heavy hardpoints allow them to mount bigger STD engines with ease. On the down side they have limited torso twist, meaning limited damage spread. Hitboxes are somewhat CT heavy, but the damage can be spread reliably with some practice. They can be brawlers, or ranged support but nowadays they're almost exclusively used for LRM boating.
Highlander: 90 tons jumpjet Assault, what else there to say? They come in all flavors and until recently 733C was the most popular mech in this game. They have almost half/half split weaponry between arms and torso. They're very cumbersome compared to Victors and Battlemasters, because of the limited engine rating. Though, they have some serious armor and damage mitigation ability thanks to their jumpjets. They're tricky to spread damage with, and your CT will get damaged much more than your ST's. They need rigorous torso twisting and JJ usage to survive for long.
Atlas: The ultimate brawler and primary target in this game. They're surprisingly mobile and agile for their tonnage and are very tough to kill thanks to their balanced hitboxes. They have great torso twist and arm motion range, allowing them to track anything at close range. They can't boat anything well, but have to build for mixed weapons. Don't let that deceive you though, they're best when they're built for brawling. Whenever you get one of these, your team expects you to break stalemates and lead them into combat. This requires you to read the enemy and your team well to prevent bad moves. They're not newbie friendly for this exact reason. Whenever you get one of these, you suddenly have responsibilities for being a damage sponge and team lead.