There is a small list of memorable songs about someone being back including Back in Black and Everybody (Backstreet's Back), while Dr. Dre has done it three times (does he have a fourth in him?)

1. I Need A Doctor (kind of cheating cause it should really be Eminem feat Dr. Dre)
2. Forgot About Dre
3. Still D.R.E.

Still v Forgot is a debate, but as far as "being back" nothing beats the intro to Still Dre.

Would you include Fuck With Dre Day or Say What You Say? How about his now hilariously infamous "Look out for Detox" on The Game's Higher?