That's a really hard question. I quite like a number of autocannon units, but, unlike LRMs, there are significant differences in range brackets and other characteristics between the different autocannon types. As such, it would be better to split it into different categories, which I will do here. I will only be working with IS chassis here; I'm sure someone else can cover Clan stuff. Additionally, I'll be trying to focus on units whose primary weapon is the autocannon, meaning it's either the centerpiece of its firepower or that it carries multiples of it.
Short range, massive damage; this is anything with "20" in the name.- King Crab KGC-005 or -005r: With two LB-20X autocannons, this has superior range to something toting standard or ultra ACs. In a C3i lance with a spotter, it's possible to achieve short range modifiers with both of your big guns while you're still 12 hexes out, which is a crushing advantage. Also, the mech has the capability to crit-seek as well as hole-punch, and it does have a long range weapon so it can still shoot when not in range for the ACs.
- Emperor -6S: Like the KGC-005, it's got twin LB-20X. However, it has NO long-range weapons, which cripples it except in city fighting, where its jump jets make it superior.
- Nightstar -9SS: does basically the same thing, but one of the ACs is a UAC/20 instead of an LB-20X.
- Hauptmann B: it functions like a Warhammer at long range, with twin ER PPCs, but gets to open up with its UAC/20 and pulse lasers at short.
- Enforcer III -6H: 5/8/5 with an Ultra/20 and some medium lasers, all in a compact 50-ton package.
- Demolisher: the quintessential area-denial tank.
- Alacorn Mk IV: 3 AC/20's and lots of ammo. Bring precision and NOTHING will want to come anywhere near it.
- Also, this list would not be complete without the Blitzkrieg, which is basically nothing more than a UAC/20 with legs. It has NO other weapons, but its speed is ridiculous (7/11) for something toting such a large weapon. With the ability to blow away light mechs in one hit and the speed to catch them, this medium mech can function as an effective area denial tool like a Demolisher, but with a much larger bubble of "you are going to die".
One might wonder why the Atlas isn't on this list. Simply put, the AC/20-toting versions of the Atlas are less than stellar in a post-Helm Memory Core Inner Sphere. Because of its slow speed, it's difficult for it to bring the AC to bear, and its firepower is underwhelming for an assault mech in this timeframe. This isn't to say that it can't still be useful, however, just that other mechs do it better. I've had a number of times where I was surprised at the performance of a 3025 Atlas that came up as part of random force generation during a FedCom Civil War or Jihad-era MegaMek game.
The same thing goes for the versions of the Victor that carry an AC/20 (though the one with all of the extra ammo bins for specialty ammo can be situationally awesome).
Medium range, high damage; anything with "10" in the name.- Nightstar -9FC: toting twin LB-10X and an ER PPC, it's extremely fast for a 95-tonner.
- Rifleman -6X: twin LB-10X in a 5/8 package.
- Alacorn Mk III: 3 AC/10's and a ridiculous amount of ammo. Load up on precision and, perhaps, a ton or two of armor-piercing or flak, and go to town.
- Templar A: UAC/10 with 3 tons of ammo to keep it firing rapidly for the entire fight, ER PPC, SRM6, and jump jets.
- Emperor -6A: twin LB-10X and large lasers. Can jump, and also has medium pulse to help deal with pesky light mechs.
- Enforcers in general.
Special mention for the Banshee -8S. It would be on this list, but it's more about the TSM and the hatchet than the LB-10X it carries.
Medium range, medium damage; stuff with "5" in the name. I'm not a fan of standard AC/5's, LB-5X, or UAC/5's, so I'm not going to bother listing them; I have no go-to units with these as their primary weapons.
- Rifleman -6D: 4 LAC/5's with LOTS of ammo for special munitions. Given that it also has a C3 slave and ECM, it's the perfect mech to deal with fast lights for your ECM lance. With a good spotter, it will always be firing at short range and getting the -2 bonus from precision ammo.
- Longbow -13NAIS: this does the exact same thing as the previous mech, but it turns it up to 11 with 6 LAC/5's. It is, however, significantly slower.
- Malice -XV: this thing is just MEAN. 4 LAC/5's with LOTS of ammo and 4 improved heavy medium lasers in a 100-ton, 4/6 package. It also has a C3 slave, though no ECM.
- Rifleman -8D: twin RAC/5's bring the pain in a 4/6/4 package. No CASE, however.
- Jagermech JM7-F: similar to the previous mech, but with CASE and no jump jets. It's light on ammo, but has a targeting computer to make its shots count.
- Ajax A: this thing is a monster with 3 RAC/5's, sufficient ammo to feed them, massive armor, a C3 slave, and ECM.
I really, really wish the Fed Suns had made a production model of the Templar (Tancred). The fact that they didn't is the only reason it's not heading the list in the "5" category. Special mention for the Atlas AS8-D. It would've been on this list, but it's more about the total package than the RAC/5 that it carries.
Long range, low damage; stuff with "2" in the name. I'm not at all a fan of any of the 2-class autocannons. They're generally not worth bringing to a fight, IMO. There is only one exception to this:
- Templar B: Somehow, this mech works. While not optimal, the 3 RAC/2's combine with the ER PPC to somehow give this mech sufficient firepower to function. It's not the most awesome of mechs, but it certainly doesn't suck, either.
Anyway, I'm sure there are other units that I'm forgetting, but these are the ones that I think about when someone says "autocannons".
