Justin Timberlake's performance in Alpha Dog wasn't just good; it was more worthy of an Academy Award nomination in the "Best Supporting Actor" category for 2006 than most of the other performances (for quick reference: George Clooney in Syriana won; Jack Early Haley in Little Children, Djimson Hounsou in Blood Diamond, Eddie Murphy in Dreamgirls, and Mark Wahlberg in The Departed were the other nominees). He was also pretty compelling in his support role in The Social Network. He was even good in the gambling thriller with Ben Affleck-- better than the movie itself, anyway.
Mos Def (or whatever he calls himself now) outshines on every other rapper-cum-actor I've ever seen by a considerable margin. He was the only good thing about S6 of Dexter. Eminem was actually the best I've seen outside of him in 8 Mile. This monologue is much better appreciated in context, but it gives you an idea of his chops: