These would be my vote:
Alacorn: 3 PPCs. Need I say more?

PPCs? ERR no thats 3x Gauss Rifles!
And for a few "canon" mods, 3x AC-10s, 3x AX-20s or a pair of heavy gauss rifles
Regulator. We seriously need more hovertanks. And who can argue with a Gauss rifle that can go 150 kph? A Heavy Gauss varient would be just insane.

In Canon theirs an Arrow IV configuration as well as a RAC-5 configuration

==============================================
A few of my choises
Houses

Manteuffel attack tank
Fed Com Fast Omni-tank
70tons
prime 1x RAC-5 and 3x ER medium lasers

J Edger
an efective light hover tank
25 tons, Medium laser and 2x SRM-2s

Demolisher II
100 tons, the bigger brother to the Demolisher, armred with a Ultra AC-20 and LBX-20, and heavyer armor (or twin thunderbolt 15 missile launchers, and even more armor...)

Galleon
30 tons, a populer light tank/scout
various armaments but most comely are a medium and paired small lasers, but others can be a medium pulse and paired medium lasers, or a TC and ER Large laser...

Myrmidon
40 tons, effectively the gold standered medium tank in the IS.
1x PPC and SRM-6

Challenger X MBT
90 tons a powerfull Fed suns heavy tank
Gauss rifle, LBX-10, LRM-10, a few lasers and other weapons
--Clans--

Shoden Assault Vehicle
Missile boat, 3x ATM-9s and 4x LMGs, or 1x ATM-9 and 4x Streak SRM-6s

Morrigu FSV
85 ton assault tank with 2x Clan ER Large Lasers and 2x LRM-15s, can be configured to have 3x ER Large Lasers in place of the LRMs

Athena
75 tons
TC equiped paired Gauss Rifles with a pair of Medium lasers and LRM-10 or the new configuration with a pair of HAG-30s and Medium lasers as well as a pair of AP-gauss and the TC (not to mention ~66% more armor)
The new configuration eats mechs alive.
though the main guns are "fixed" to the front
I think that would do for now, a few others I would not mind seeing have been suggested by others as well.