I've been told that
HQPlayer is the best, although there is a co$t. One thing that it does very well is upsampling, assuming your DAC is able to the higher rates and resolutions. There's a desktop version (as linked) as well as an
embedded version. You can use the embedded version with many other players (and
piCorePlayer), to inject its upsampling ability.
Fortunately you can run it in trial mode to see if you appreciate the effect, before putting your money on the barrelhead.
As for the benefits of upsampling, I'm a recent convert. I figured that diddling with the data stream could only make it worse, but it turns out it really does sound better. I'm currently using a piCorePlayer with Logitech Media Server, using the
built-in upsampling features. It really did add a surprising level of realism and sweetness to the overall presentation. At some point I'll try HQPlayer embedded, to see if it's enough better to warrant the cost.
BTW, when using upsampling, it's generally suggested that you run your DAC in NOS mode, although not all DACs support that, even if they say they do. (I'm currently using a Denafrips Pontus II, which has an NOS switch, but apparently doesn't actually do NOS.) I'm considering upgrading to a Holo Audio May KTE, although I have other things in the pipes first.