I think the Samsung P3 is the only player that fits all of your demands since I just read that it also supports audio streaming over bluetooth. I hesitate to recommend it since I haven't heard it myself and it's pretty expensive. But if you search you'll find that on some specialized forums people rate the SQ of it's predecessor (P2) highly, on par with Sony.
How important is wireless streaming to headphones to you? Not only does it limit the choice of players, perhaps more importantly: it limits your choice in quality headphones. Ofcourse the Samsung will be good with a wired headphone too, but then Cowon's and Iriver's players are more affordable.
I assumed FLAC support was a hard demand (since I really appreciate FLAC support too), but if you're willing to transcode FLAC files, like Beta4Me suggests, there's lots more possibilities. You could get a Sony which is excellent, or an iPod Touch. I wouldn't recommend an iPod over the others I mentioned for SQ (Mind you the Touch's lesser SQ might not matter if you're not going to get good third party headphones anyway). But the Touch does have redeeming qualities like a good GUI, wifi and a rather good internet browser, which is very neat. The Touch is also very cheap compared to Sony or Samsung. Oh and it looks pretty ofcourse.