For much of the '90s Robert Downey Jr. was notorious for his addictions. But having clawed his way back to the top of the A-list at 43, he's not apologizing for the one bad habit he has yet to kick. "You don't inhale," says the actor, as he puffs contentedly on a cigar during a recent interview. "Maybe I'll get mouth cancer, but I doubt it. I'm hard to kill."

Having survived drug abuse, rehab and prison, Downey has proven himself to be nothing if not indestructible. And thanks to the massive success of his new movie Iron Man—the film made $100 million its first weekend—his career seems as superpowered as the hero he put on 20 lbs. of muscle to play. "He's on fire," says Iron Man director Jon Favreau. "There's no stopping him." Adds Ben Stiller, who directed him in the August comedy Tropic Thunder: "He is at the top of his game. He has been through a lot, and his priorities are pretty clear."

For too long other priorities led Downey down a darker path. The son of film director Robert Downey Sr. and actor Elsie Ford, Downey dropped out of Santa Monica High School to act, landing a gig on Saturday Night Live at 20. A slew of roles in movies like 1987's Less than Zero followed, and in 1992 Downey landed the title role—and an Oscar nod—in Chaplin. But his struggles with drugs eclipsed his talent. In 1996 he was put on probation after an arrest for driving in Malibu with heroin, cocaine and a concealed .357 Magnum. For the next five years he was in and out of rehab and jail, doing time in California's Corcoran State Prison in '99 for failing to take a mandated sobriety test. In '01 Downey was fired from Ally McBeal after another relapse.

It was a wake-up call. After another rehab stint, he slowly found his way to health by practicing yoga and kung fu, and by finding love with producer Susan Levin, 34, whom he wed in '05. "She's fantastic," says Downey, who's also devoted to his son Indio, 14, with first wife Deborah Falconer. "I'm not a walk in the park, and [Susan's] a very complex and engaging person." Downey, says Stiller, "talks a lot about how his wife and his son are his grounding forces." Today his strongest drink is black tea. "My vice, it seems now, is creativity," he says. "It's all about living a normal, balanced life."

Except when it comes to attacking his film roles. To get abs of Iron, Downey endured daily rigorous workouts that continued during 14-hour days on-set. "Just pumping, pumping, pumping," says his Iron Man trainer Brad Bose. "It's not for the faint of heart." And, adds Downey, "I'm no strapping 20-year-old."

Nope, just a guy with a career younger guys would kill for. In addition to Thunder, Downey will star opposite Jamie Foxx in the drama The Soloist this fall, as well as the Iron Man sequel due in 2010. Favreau says his star has proven he's a true man of steel. "Nobody appreciates this opportunity more," says the director. "To have another shot at this, after what he's had and lost, is as redemptive a story as the movie itself."