I suppose it depends on what you mean by 'powerful'.
Rockbox, Vibez, Karma et al have a heavily-featured EQ, but what makes me laugh certainly about the Rio-derived products, and perhaps at those who proclaim these as the portable EQ to end them all is that what they did to the sound wasn't actually much good. Rockbox is not a bad implementation as such if the Karma placebo turned you on, but the level to which you have to fiddle seem to me to be counterintuitive, especially as there's no way as far as I know to import the presets. And as for the effect on the sound, it depends on the player and the level to which the EQ is being worked.
Cowon and Creative have what could be called the start (Creative) and the end (Cowon) of the middle of the road in terms of decent control of the effects, but EQing definitely has a negative effect on the SQ - more so on the Creative than the Cowon IMO.
Sony players have a simpler, but more intuitive EQ which doesn't go overboard for a portable device, and IMO they retain their musicality / quality better than the above players even when pushed fairly hard. If you intend to run your player with FX / EQ switched on all the time, I'd be inclined to recommend the Sony S-series. They are not the last word in quality, but are definitely good enough to run at high-bitrate MP3/AAC with the effects switched on, and still remain very listenably musical. If you just want something featureful but decent quality with a touch (or a serious daubing really
) of nerd, then the D2 still seems like a very good bet.
As for 10-band EQ's etc, I'm obviously speaking in a den of nerds, but even in that case I think you're way overnerding this stuff.