Three things I would like to know. How much are you willing to spend, storage and do you want your MP3 Player to be a pure music/multimedia player.
Personally I have a 16GB Creative Zen with a 16GB micro SD card but I find my self using my HTC Desire (with a 16GB Micro SD) as an MP3 player as it means one less thing to carry.
If you go down the Galxy S2 route, you can get a 16GB for a decent prive on Amazon or pay extra for a 32GB card.