Hands down, the best music player I've tried is 'Music Player Pro'. I actually wrote about this on another thread and am working on a large review of it. I've used all the major music players at some point, looking for that one cure-all solution. bTunes was my favorite for awhile, however since the developer abandoned it, i've moved on knowing that it's bugs are only going to get worse with new builds. MixZing is very feature rich, but the interface is boring and not very intuitive. TuneWiki has some really unique and cool features, but still isn't ideal. DoubleTwist looks nice but you can't even create a fricken playlist on the fly and there are some other standard music features it lacks.
However, a few weeks back I found Music Player Pro which had just recently released and I can finally say that I've found a rival to iphones music player. Not only that, but the developer is EXTREMELY responsive. Just a week back, people were asking for the ability to scrobble and the developer already added it.
You can download and pick album artwork for albums, search for lyrics, retrieve artist info, use various skins, search library by artist/album/song/genre/playlist/folder. You can create playlists on the fly. You can filter out songs by keywords. In the future the dev is adding the ability for custom skins, editing ID tags and more..
Also, the current skin is sorta meh, but a new skin will be launched in the next week or so that features custom vertical and horizontal layouts.
How do I know this? Well, having been so happy to finally find a great music player, I created a skin for the player and have been talking with the developer ever since and he is working on integrating it now. I know I probably sound like I'm advertising, but honestly, I've been in the same exact boat as all of you and I won't be making a dime for the skin. I, as you probably have, have just been looking for that music player that 'does it all', and I'm just excited to actually find one that is not only great already, but has so much potential to be greater. As I said before, I thought bTunes would be that player, but with the developer abandoning it, I moved on.
I'll post some screenshots of the updated skin here a bit later.