I've been playing Battletech/Aerotech since the late '80s, and can remember when the only autocannon was the Autocannon (AC/5, which is why so many early 'mechs from the Shadow Hawk/Wolverine class up to the Marauder mount a fairly crummy weapon,) how the Warhammer and Battlemaster reigned supreme, before extra-light engines, Endo-Steel, and Advanced Tactical Missile systems. I love the new developments, don't get me wrong, but classic old 'mechs like that Shadow Hawk wouldn't last 3 rounds against a fellow 55 ton Clan Omnimech.
My favorite new weapons would be the HAG/40 (bad ass! except for the sheer tonnage you have to set aside for ammo) and the Heavy laser series, especially the Medium Heavy Laser. It's like having a 1 ton shorter-range PPC with less heat. Mount a computer if you don't like the -1 targeting modifier. I enjoy the myriad ECM/Active Probe/C3/Targeting Computer developments too. Least favorites? The AC/2 and pretty much any Small Laser. Except for very specific anti-aircraft or anti-personnel situations, the AC/2 is a waste of 6 perfectly good tons for extreme-range MG-like damage, and the range and damage on the Small Laser make it little more than last-ditch weapon. Mount more armor in its place, I say.
A fun and easy Assault 'Mech design: Strip down an Atlas, drop in a 300 XL engine (keep the standard internal structure) and a pair of HAG/40s with lots of ammo plus a 6 ton Targeting Computer. It eats armor for breakfast, and can reach out and touch someone from far away. Just don't run out of ammo! I need to check the criticals on that, come to think of it. And there might be a better way to use 6 tons than in a computer, but what a great direct-fire support 'mech. Like a Rifleman on steroids.
And yes, I do appreciate the finer things in Battletech - while I listed nothing but heavy weapons and Assault-class 'mechs above, in my book nothing beats designing a speedy 40-45 ton raider or a tough, well-armed, well-armored, mobile 65 ton bruiser.