If you are honestly considering this level of vehicle, you really need to go drive them each. They feel very different. This generation M3 feels like a boy racer gone wrong. Loud, racous, edgy, very fun but it ALWAYS feels like an M3. I don't know about the C55, but the E55 feels like a Mercedes that transforms when you press on the accel. The M5 would definately be my choice but then, I'm a BMW guy through and through. They are all nice cars, but the E55 and M5 feel the most stately when not hammering it while like I said, you never forget you are in an M3.
About three weeks ago I was talking with a guy that had a brand spankin new C55. He had previously owned a second gen M3 and a current M3 and he said that other than flying through the turns, he preferred the C55 to any of the BMWs. I'm 34 and he was probably in his early 40s though so perhaps his edges were a bit softer than mine.
Honestly though, the GS300 looks nice, but it isn't the quickest car around. A G35 with a manual feels like a rocket in comparison and you won't have nearly the shop charges compared with the BMW M or AMG cars.