I find the hardest to cut out of my language is dumb, because no one else seems to realise it's problematic. While retarded and lame and gay are common in the general population, they aren't among people I hang out with, but I have never seen anyone besides myself bring up dumb. (Though it's also hard because it feels "lighter" to me than stupid, so substituting stupid doesn't always have the same feel to me. I still manage to avoid using it probably 98% of the time, though.)

I didn't even realise cretin was problematic. (Not that it's a word I've ever used to begin with.) Same with spaz. I only realised a few years ago that it came from spastic, which referred to people with cerebral palsy. The only way I'd ever heard it used was to mean airhead, so I had assumed it was a made-up word.