I'd have to say that Rangers, Tru, and Spec arms are the best. I've dealt with SD-Unit too but not anything major (mags and such). They handled themselves well, but recently they've lost their website and haven't been updating whats left of it lately. I'm a little sketchy about DEA since their inventory is similar if not the same to Rangers and so is their site. Seems fishy. But they have a rep while I wasn't present here so I'd give em a go if they had something I needed. I also like that rangers has 4% tax, only really major when your buying 500 and up or so.
Also 007 has what looks like a creditable website just haven't tried them yet.