What makes a 'Mech fun? Not so much when it wins, but when it keeps you from losing.
Everything's gone to hell in a handbasket, your main force is all still trying to stand up on one leg so it can limp off the field, and there's this one Jenner or Phoenix Hawk or similar mech bouncing around going "look at me, look at me" as it keeps raining destruction on a larger force.
It's the Guillotine that wouldn't die, and survives the enemy's best attempt at an ambush, limping off the map with a gyro hit, 2 engine hits, and only two points of internal structure remaining in its center torso.
It's watching your opponent struggle to adapt when you're playing on an open terrain map, and your demi-company of Lightrays starts imitating mongol tactics. (My players hated my guts for that, but not nearly as bad as when I discovered the Preta...)
It's the legless Vindicator that props, fires, and lands a single LRM on an approaching, fresh Cauldron Born and crits its engine out. (Wasn't my story, happened to a friend).
It's the Savannah Master that took down a Condor all by itself through nothing more than guts, persistence, and luck. Vehicles can be fun, too.
It's when your opponent accidentally leaves his mech one terrain level below yours in a place where you can run up to it, and you kick his head right off, imagining a couple of trees as goalposts...
It's seeing a force that you know can still beat you surrender or withdraw because they think you're doing a lot better than they are, when all that's holding your mechs together are wishes and force of habit.
It's all the little moments when, by some combination of unfathomable luck, tactics, or just bloody-minded persistence that you turn an inevitable failure into something epic. Just about any mech can be fun.