Best sounding mp3 player?

Best is very subjective. A lot of people like the Sony NWZ-A8xx series, Cowon D2, iAudio 7, Sansa clip, and even the iPod is extremely popular. If really depends on what you're going to use with it (headphone, etc).

I think the Toshiba Gigabeat T400 sounds a bit better with most cans than the Clip. The Clip is a notch better than many players, but the T400, set to no EQ, sounds better by a small margin. It will not drive high imp. phones, but the SQ is top tier. At $80 for a 4GB player it is a steal! No FM though, if that's important to you. BTW Toshiba has not put the same H2c sound enhancement firmware in the Zunes. How much of the sound quality is hardware or H2c I don't know, since it apparently tries to restore some of the fidelity lost in MP3 encoding. I listen with the following without external amplification.
Alessandro MS1
PX100
E4c
KSC75
Testing my PA2V2 now, but don't see an appreciable improvement is quality.

what type of music do you listen?
what files type are you going to play most?

There's no "best", it's all subjective.

I originally had a Creative Jukebox 3 that I thought was good.
Then I got the Creative Zen Vision:M which was OK but not quite as loud or as vibrant as the JB3.

I heard good things about the Sony NWX-800 series so I bought one and I must admit I was impressed. The things with the Sony is that the EQ effects actually make a significant difference to the sound. You can also tailor the sound with your own "user settings".

There are various reverb/ echo effects on the Sony and again I found that on certain tracks this definately fills out the sound and makes it better.
The matrix setting makes acoustic music sound more realistic.

The other thing with the Sony is that the volume goes very loud (in fact it's unbearably loud on the max setting).

When I bought a Ipod Nano 3G the music was much quieter and not as vibrant. The frequency extremes seem to be rolled off on the Nano.

I was listening with Shure E4c's and after 2 weeks of solid listening I was dissapointed with the Nano 3G and was thinking of giving it away.
However I decided to buy the UE 10 Triple.fi and I can't believe it.

The Nano 3G now sounds vibrant and detailed. The triple.fi specialize on low bass and high treble, this was missing on the Nano but now its all there and I'm now much much happier with the Nano 3G.

Conclusion:
1: The ear phones make a hell of a difference to the SQ.
2: Players with louder sound volumes can be percieved to have better SQ.

Another factor is that (within reason) your ears adjust to the SQ of the player. You don't realise how good (or bad) your MP3 player is until you compare it to us.

I know plenty of people with Ipod Nano's and the stock Apple ear buds and they are very happy with the SQ, but they've never heard anything else.

Have you tried your UE10's with the sony walkmans?

Meizu M6 has SRS WOW and is the best sounding portable player you can buy.

for your info, any SRS makes all the music sound UN NATURAL which is also not preferred by audio purists or audiophiles, SRS does not make the best sounding player just because it has SRS on it. why dont you try listening without SRS and see if the sound is all balanced across the specturm

Iriver T60 has a very good sound.

We can't recommend you anything unless you give us some information about your listening habits.

SQ = Sound Quality
SRS = A company that makes audio technology

A lot of people think (myself included) that the ipod, by itself, is average in terms of sound quality. If you use an external amp through the line out, then it is among the best sounding players out there.

I like the sony nwz-a816, but there is a slight hissing at low volumes.
The iriver clix2 is commonly referred to as the benchmark for SQ.
Loudness only improves the PERCIEVED quality of sound, and true SQ is trivial for the upper tier of DAPs IMO. anything better or equal to an ipod (all mentioned in this thread so far) will sound great with great headphones and good with good headphones and not so good with... you get the picture.
So, yes, invest in a good pair of phones and pick a player you like that has gotten good reviews as far as sound (or that you have heard in person) and that you like the interface and design of.

Have you tried your UE10's with the sony walkmans?

No I haven't. I lent the Sony to a friend and haven't got them back yet.

I'm keen to get my hands on the Sony to see how much the triple.fi's improve them. I think synergy plays a big part, some ear phones suit certain mp3 players.

As I said the thing with the Sony is if you find the bass lacking then just turn it up, if the mid range is recessed then just turn up the mid range frequencies.

When I use the graphic equalizer on the Ipod nano it seems to make the sound a little worse. The increase/ decrease in frequencies isn't that noticable but the sound seems to break up and be more distant.

Using the graphic equalizer on the Sony has quite a big effect and it's easy to mess around with the settings to improve the sound on certain tracks.
The reverb effects have stadium and cave etc, most of the settings are gimmicks as they have too much echo making the sound worse. However there's one reverb setting called "matrix" that only gives a minor echo effect and on simple acoustic music and vocals its makes it sound more real.

But we all have different preferences. You buy what you enjoy and it doesnt matter what others think. As I've said some people think the Ipod nano 3G with the stock earbuds is brilliant SQ, but I know different.

I listened to the Nano 3G with stock ear buds for 2 full weeks and couldn't get use to the sound. I wasn't enjoying music as much as I normally do.

How about Kenwood MediaKeg 60GB one?