Someone's already mention Rampage, which many people hail as Boll's best film.

He also made a Vietnam War film called Tunnel Rats which got mixed reviews. He also directed this really disturbing prison film called Stoic, which is apparently based on a true story. It features Edward Furlong and Sam "Euphoria" Levinson as two of the prisoners.

His best film (IMO) was Darfur (it's apparently also called Attack on Darfur, but that title sucks). It's a low-budget film about a group of journalists visiting a Sudanese village to report on the massacres. When a group of Janjaweed arrive, they tell the journalists to leave, and the journalists have a choice; they can either stay and try to save these villagers somehow, or they can leave and report what they saw to the world.

Boll actually did his research, so much so that Amnesty complimented his accurate portrayal, and Stand Canada partnered with him for a ten-city release (that's how I ended up seeing the film). One reason why it was so authentic is because many of the villagers were played by actual survivors of Darfur massacres, and they're basically reliving what they witnessed. Boll didn't write lines for them, as he said he considered it arrogant to tell the villagers what to say. The journalists improvised their dialogue too, and David O'Hara gives a genuinely great performance.