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!