5 best songs and 5 best for Rock Band will be two different lists...
Personally, if I were only to get 5, it'd be:
Under the Bridge (for popularity/familiarity)
The Greeting Song (for challenge, most likely)
Breaking the Girl (for vocals, great bass line, but mostly strumming guitar...a nice change of pace)
Mellowship Slinky in B Major (without a doubt, one of the most interesting progressions on the album)
Suck My Kiss (one of the most straight-forward tracks on the album, nice parallel guitar and bass lines)
alternate:
I Could Have Lied (slow, mellow, some awesome melodic solo work, same with the vocal lines)
Sir Psycho Sexy is a cool track, but it's 8 minutes of a very slow repetitive jam. Awesome for musicians, potentially mind-numbing for gamers.