I have several, all on bass:
Queen-Another One Bites the Dust. Not too bad. Just need to work on making it smoother.
Coheed and Cambria-Blood Red Summer. Another easy one.
Coheed and Cambria-Devil in Jersey City. Mostly easy with some difficult bits to work on.
Rush-Limelight. Not too difficult, not too easy, some parts in the solo/near the end giving me trouble, but I'm having loads of fun with it.
Iron Maiden-Aces High. Told I should learn this from one of my friends (who knows it on guitar and thinks we should combine talents). Obviously, its hard, but not devilishly hard. Hope to actually seriously try it in a month or so.
Rush-Cygnus X-1. Just doing the instrumental intro for fun sometimes.