Best:
1997 Jack Nicholson As Good as It Gets
1999 Kevin Spacey American Beauty
2003 Sean Penn Mystic River
2004 Jamie Foxx Ray
2005 Philip Seymour Hoffman Capote
Of these, I'd say Hoffman's performance was the standout.
Worst:
2000 Russell Crowe Gladiator Maximus Decimus Meridius (ironic that Crowe should get an oscar for what was arguably his worst and least believable performance)
2001 Denzel Washington Training Day (Washington added no new dimension to the stereotype)
2007 Daniel Day-Lewis There Will Be Blood (way over-the-top)
2008 Sean Penn Milk (a political award that had nothing to do with Penn's performance, which was good but not great)
2015 Leonardo DiCaprio The Revenant (the make-up artist and cinematographer deserved the award, not DiCaprio, who mostly just walked around in the snow)