I've had a hard time finding a good class mod. In the first game, it was easy to find a good one, because they all benefited you. In Borderlands 2, a lot of them have negative effects, and there doesn't seem to be a clear cut best. On my Siren, I'm using the Fire Rate class mod with Kinetic Reflection. I recently found a level 44 one with Kinetic Reflection and something else, but my siren is only level 30.
My Mechromancer uses the Prodigy (+reload speed, +accuracy) with the fire rate and health regen talent. I have another one similar to that with better stat bonuses, but it has the fire rate and elemental effect chance boost instead. I rather like the health regen talent, so I'd rather not lose it. The Anarchist class mod may seem like the best, but I think the talents that it boosts are just awful.
Yeah, the HOT loaders are the worst of the bunch. For some reason they just rip through me. There are a couple loaders that are just ridiculously durable too - like the ION Loaders.
I've found that Hot Loaders aren't too bad if you strafe the whole time. Their attacks seem to be more like napalm as they have a pretty decent AoE radius, which is why always moving sideways works well. Ion Loaders seem to lack the joint crit points from what I can tell, and there doesn't seem to be a way to break their shield? I mean... you can run in there, but it will deal shock damage to you, and shock weapons don't do anything.
I don't really have much issue with any of the loaders now. PWR Loaders are annoying if you don't get them right away because they deflect your shots (though as a siren that's easy to mitigate), so you have to aim for their feet. I pretty much always use corrosive weapons against all the loaders, so that makes it easy anyway.
If I remember correctly, an explosive weapon or E-Tech corrosive will work well against a Pwr Loader. Explosive will explode when it hits their spining arms and still deal damage to them, and I believe corrosive will still deal damage as well. I know that I've just swapped to my Corrosive E-Tech SMG quite a few times when they popped up. Although, that's a level 34-or-so weapon, and it's getting a bit long in the tooth at level 44!
The key with ALL the loaders, especially WAR and RPG Loaders and Badass Loaders is to hit the crit points early. I have this one pistol which does 400% crit/corrosive damage, so it gets kind of ridiculous in a hurry.
Yeah, hitting crit points isn't that easy with a Mechromancer with 300+ stacks of Anarchy.

I do end up hitting the crit points with a rapid fire weapon, but it isn't too often since the bullets fly all over. I recall a few fights (the game threw quite a few Super Badass Loader + Badass Loader + Super Badass Shielded Surveyor combos at me) where I knocked off at least one arm off of them before they died.

That pistol sounds rather nice!