Well this depends manly on your mech you are driving.
My Jenner has almost maxed armor exept some points from the arms. Legs max since its get shooten the most.
My other mechs also run near max armor with some stripped from the legs. Usually down the next round ton to max loadout. Cant remember being legged in a non light once. At least not when im not dead anyway.
So legs are still the best place to put ammo, since CASE is useless with an XL.
Also i never go for the legs, unless its a light or already beaten up. CT kill them way faster. And yes, please try to leg me in my DDC, i will thank you in advance.
And there was this glory moment in my Jenner: Base defending against 2 others, im the only one left and clock is ticking down. I manage to kill one, but i am badly beaten up. The other one (a catapult) legs me and is 20m next to me. I circle around, aim for his head and boom, alpha-onehit-headshot, match won.
He would have killed me if he just have aimed for my CT though.
So allways try to kill not to criple. A wounded mech still hurts badly