I'm not a violent person, but there's something about James McAvoy that makes me want to give him a good slap. (I didn't quite consider Atonement to be a feelgood movie, but I was probably less depressed by the ending than the average viewer.)
Here, "Slappy" McAvoy plays Dr. Nicholas Garrigan, a young doctor fresh out of medical school who desperately wants to get out from under the thumb of his stuffy family. Unfortunately, it's the 70s, and he picks Uganda as his place to go out and do good. (He also has a serious case of can't-keep-it-in-his-pants-itis.) Via a series of unlikely occurrences, he finds himself in close orbit around Ugandan dictator Idi Amin. At first he's charmed and persuaded by Amin's huge charismatic personality. Only later does he become aware of the flipside: Amin's also a homicidal maniac. Can't have everything.
Forest Whitaker plays Amin, a role for which he won many plaudits, including an Oscar for Best Actor. Well-deserved.
Although the movie's obviously based on actual events and people, the Garrigan character is largely made up; Amin did have a close relationship with a white advisor, but the advisor was even less sympathetic than Garrigan. Cracked (my usual source for insightful movie criticism) has the scoop, where The Last King of Scotland is number 3 in their article "6 Movies Based on a True Story (That Are Also Full of Shit)".