there are no best software, it simply based about your features required, which would been your best software. It also based how you have your music collection masked (how are they sorted in your harddrive? Do they have tags? Do you have singles and albums, or both? and so on)
Try few one in the wikilist, and then take one you like most.
Personally my favourite competer software is property Sk-Jukebox (discounted, but it have very good well designed GUI and features), but many here also like Freebox (shareware) and Uncle'T Jukebox (freeware).
Download album arts is not a major feature in jukebox software, but there exists nice external to do that job. Hence it not a feature you see in most software, also no mine. But of course they would show them.
Typical they named folder.jpg, but M$ have to hide the folder and hence it might not work in some software (why Mircosoft?), hence I in my software also support cover.jpg as well. Many software just use the first picture they find in a contain folder, if folder.jpg not found. But they do nearly never read coverart from a mp3 file itself.