Of the mechs I've mastered:
My Splatcat in closed beta. Maybe it was bad hitreg, or bad players (or both). But I would also have all my armor gone before I would die from any side torso dieing. I had a few times where I would have 3-4 players chasing me, but I would be too fast due to 315 XL engine and Jumpjets (there's a lot more fast mechs on the field now). A few times some people in the chat said "WHY WON'T YOU DIE" or "HOW ARE YOU STILL ALIVE!?". Anytime I got Splatcat on River City it was practicly a guarunteed win.
I've only piloted it a couple times recently since I normally just focus on leveling mechs, and I only ambushed a few times in the times I piloted it, so I didn't really get shot much. Still can 2 shot an Atlas from behind to strip it
Blackjack is another mech that makes me feel unkillable, but situationally. Give me a good position where I can hill-hump, and the enemy will never be able to shoot me because I expose so little the enemy ends up shooting the ground 95% of the time.
Been using the Centurion as a zombie. 3/4 of the time I feel like I last forever, the other 1/4 of the time is when I get overconfident and get my face stomped on (usually dieing from the front CT, when I'm facing away from the enemy).
I want to say Thunderbolt because those shield arms are so sweet. Though saying the Thunderbolt feels unkillable is a bit of a stretch as it's too slow, and it's weapons aren't that impressive to me.
The following feel like they die easily to me:
Atlas: Too slow to turn it's torso, and it's easy to hit whatever you want on it (Don't blame it on skill either, I've never had a problem hitting enemy atlases wherever I need to either)
Cataphract (especially 4X): Easy to hit side torsos (At least for me, seems not to be the case for most people strangely) + Center torsos, and is generally slow. Probably a good thing because of their damage though
Wolverine: My legs, WHY DOES EVERYONE GO FOR MY LEGS IN THIS MECH!?!?! Also it always felt like my legs got 1-2 shotted in this mech too (I didn't have legs in the ammo either).
Trebuchet: Size of an assault, easy to hit stuff, with the armor of a medium. Due to the gigantic size I basically made this mech LRM focused except for the 7k (which I could hillhump with somewhat decently due to rather high hardpoints).
Kit Fox (AKA Mobile ECM Assault support): 20 Damage alpha isn't exactly impressive, especially with how slow + Lightly armored the Kit Fox is, I was basically just hanging near allies hoping to be unnoticed (which works 90% of the time)