OK, IS Lights...
Let's go A-Z (or A-W)
Anubis
A 30 tonner with ECM, energy arms and missile or energy torsos. Possibly redundant with the existence of the Javelin JVN-11B, but I think the arm mounts make up for it. Those missile racks could be an easy target, however.
Garm
A 35 tonner mixing ballistics & missiles, energy & missiles, or ballistics & energy. All torso mounted with a lovely cockpit height mount in the side torso. Would need hardpoint inflation.
Hollander
A 35 tonner with a cockpit height ballistic mount. No JJs. No ECM. No MASC. No backup weapons
The only other available variant adds 2 energy hardpoints. Honestly, it would need serious hardpoint inflation and it's probably less effective than the Garm.
Jackal
A 30 tonner with energy & missiles. Cockpit level side torso energy hardpoint. A variant comes with ECM. Needs hardpoint inflation.
Koto
A 25 tonner with energy weapons. One variant has ballistics. High arm mounts. Base model is not legal, due to X-Pulse technology.
Mercury
A 20 tonner with energy weapons and MASC. Good possible counter to the Fire Moth (Dasher) if it ever gets put in game. Squat profile should be good for cover, arms would need to stick straight out to be considered high.
Raptor
25 ton OmniMech. All weapon types. Great mounts, good profile. Possibly too slow with the locked 175 XL engine (Sarna lists 118.8 kph, Smurfy lists a Commando with a 175 as 113.4 kph). One of the better IS Omnis, in my opinion. To be totally honest though, I prefer the
Raptor II
Stiletto
A 35 tonner with energy and/or missiles. Basically a Jenner with (hopefully) better hitboxes. Also has an ECM variant, but I think it has arms for... reasons?
Thorn
A 20 tonner with energy & missiles. A variant has MASC for another possible counter to the potential Clan Fire Moth (Dasher). Energy hardpoint just below the cockpit (Head or CT?) High mounts for the right arm. I love the asymmetry of the arms.
Wight
A 35 tonner with energy weapons. Mounts just below cockpit. Has JJ variants, but only one is legal. One year ahead of the timeline jump.