Guitar Hero 5

This is the easiest way, not the fastest. And it is used to get more than 505 stars too.

Play through the career on beginner vocals. Just put your mic in front of a fan or something that makes constant noise. This will get you 6 stars on everything except for the two songs without a vocal part

Now do the challenges which unlocks cheats - Auto-kick for drums, No-fail, Always slide and all notes Ho/pos.

Whammy and starpower challenges can be done on easy or medium, because the same amount of opportunities come on these difficulties..

Dont do the band challenges as you should be able to get more than enough stars this way.
Usefull cheats:

-Always slide(all notes are tap-notes) : Get Platinum on Du Hast

-All notes ho/pos : Get Diamond on Hurts So Good

-Auto-kick: Get Gold on Mirror People

After unlocking the cheats, just apply them before doing challenges. You apply cheats at the "character select"/"band" screen under Options->Cheats.