The UE 6000, which is currently at closeout prices--that fall into the OP's budget--is certainly a value purchase. The 6400 is also a good can; perhaps more durable but (by objective measures) not up to the normally 2x the price UE 6000 in sound quality. Of course there are many other good cans in this price range.
But whether "it" is the best in the gym; best for a particular sound signature; etc. is a matter of both what factors are considered (most of which haven't been addressed) and what personal preferences (e.g. preferred sound signature) are. How important is durabiity; how important is weight; how important is isolation; how important is low sound leakage; how important is e detachable cable; how important is high efficiency; how important is...?
Well, Im very newbie to all this, so not sure what you mean by sound signature.
In regards to other questions: Im looking for well balanced sound, nothing to bassy, nothing to high on mids. Just great quality sound. I want something that is comfortable and durable. Noise cancellation/isolation isnt huge for me, as long as there is some with the over ear cupping (and is comfortable). Dont want them to be too massive and bulky, so maybe the smaller the better (but isnt really a deciding factor). Detachable cable not important. Dont know what you mean by efficiency..
I was reading the UE 6000 reviews and I noticed a couple people saying they started hearing white noise and crackling soon after purchase.. This makes me weary of buying, but of course I wanted your guy's opinions on this first.
*edit: Also, are there any shops in NYC where I can go to listen a selection of headphones? (sans best buy ofc, or any other silly chain)
You would use the UE 6000 with active noise cancellation turned off (it sounds better that way)--so no problem (even if it is a problem).
The KRK should also be good, especially for gym use (although they are likely to get hot on your ears as they are over ear--not on ear).