LAMs with pulse lasers. I had a very traumatic, death-by-1000-papercuts battle against one. My ultimate custom 3 BFG 100-ton 'Mech was slowly gutted from the back to death by a LAM with pulse lasers.
I had agreed to the battle misunderstanding that pulse lasers were -1 to hit (a rulebook review ended that hope) and assuming a few hits from my Gauss rifles and UAC/20 would end it.
No, when the LAM lost initiative it ran and hid behind hills, which I couldn't lumber to fast enough. When it won, it would jump behind my back and I only had one arm-mounted BFG (a Gauss) to wave in its general direction. But its movement mods and my minimum range meant I couldn't hit.
Modern rules have nerfed a lot of the jump jet and LAM abuse, but that secured my LAM hate.