well, we know people like to throw random things in when they decide to reissue an album with a european, japanese or just plain simple bonus track, just to make loads of cash so generally, these are more commercial or just plain bad.
but for me the exceptions are:
nightwish-sleeping sun(wtf??? how can they think not to put this on the oceanborn cd?? it's probably their best ballad ever, and that's saying a mouthful, and the song below is probably their best "heavy" song, and that's not saying much lol but it fucking delivers)
nightwish-nightquest. this is just brilliant, a fucking heavy riff, great piano solo, and it kind of reminds me of priest's electric eye in a way..

nostradameus-scream of anger
hammerfall-detroit rock city
both are covers, both are just fucking awesome. why oh why didn't hammerfall just forget the other stuff, that song is so awesome i didn't even hear the other bonus tracks and i can surely say it's probably the best one.

judas priest-hellbent for leather(79): let's see now: priest+hellbent for leather+1979+live+unleashed in the east+rob FUCKING halford+tipton and downing=

out of the list, i choose sleeping sun.