Great question, nalyd. raleh made a great point. You have the consensus top three centres in NHL history: Gretzky, Lemieux and Beliveau. Then you have the next six - Clarke, Messier, Trottier, Mikita, Esposito and Morenz. Outside of Morenz and Espo, they're known for their grit and physical play along with their skill.
I'll go with Clarke, but I don't blame anybody for choosing Messier. I loved watching both play - despite their penchant for cheap shots - although both have had their iffy off-ice moments. (Especially Clarke as a GM). But I respect both. I'm not a Messier fan in the least, but he definitely earned my respect.
I would say that Messier was the better goal scorer, but Clarke was the better playmaker. They were both outstanding in the playoffs. Messier won more Cups, but keep in mind that Messier was on the dynasty while Clarke ran into the dynasty. They were both very physical. Messier was more likely to plough into an opponent, but Clarke was no spring chicken in that regard, and Kharlamov wasn't the only victim of Bobby's stick.
Clarke was definitely the better defensive player. He won a Selke and probably would have won more if it existed before 1977. And as much as we've all heard about Messier's leadership ability, Clarke was the better leader. Clarke might be the best leader of all-time outside of Jean Beliveau.