There's a crap ton of megamek options. Some noteable ones:
StratOps Quirks - enables quirks within MegaMek. Gives individual mechs a little more 'character', e.g. the Stalker has the combat computer for extra cooling, Orion hits aircraft easier, etc.
TacOps Enhanced Missile Defense - lets your AMS totally shoot down a missile salvo if you roll high enough
Through-armor Criticals 'Float' - normally, every TAC is against the center torso. Now it's not. Reduces the amount of lucky engine/gyro kills, but now no ammo bin is safe
TacOps starting fires - pretty self-explanatory, can start fires intentionally for smoke cover or to smoke out infantry
TacOps sprinting - sometimes you need to move really fast and don't care about shooting
TacOps backwards movement - can climb backwards over hills
TacOps attack physical PSR modifier - lights have an easier time landing kicks than heavies/assaults
TacOps hulldown/falling expanded - if you "just" fail your PSR by 1 to remain standing, you go hull down instead. You can also voluntarily go hull down, which is great when you're hiding out behind a short hill, as it gives you an extra cover bonus
StratOps advanced atmospheric control rolls - slightly fewer lawn dart checks for your aircraft
Advanced Aerospace Rules / Ammo Explosions - aerospace fighter ammo can blow up
RPG Related / Pilot Special Abilities - can use pilot SPAs in MegaMek (e.g. dodge, gunnery specialization, etc)
There's a lot more, but those are some of the ones I like to use.