US Football - Bryan Westbrook. He's an alum of my high school, as well as a guy who made it big time despite being played off as too small and being picked quite late. He has been able to carve out a role as an elite receiving back.
Baseball - don't watch enough to have one
Hockey - Dominik Hasek, I always loved watching him. Completely flaunted Roy's butterfly for a unique style often described as "flopping around like a fish" and became one of the greatest goalies of all time. When he played for the Sabres, it seemed like every year he almost singlehandedly won them at least one playoff series.
Tennis - Nadal, I like his old school defensive game. He's the antithesis of someone like Roddick who has a monster serve but isn't really a truly great tennis player otherwise.
Soccer - don't get enough of it in the US. I loved watching the German team's aggressive and open play in the 2006 World Cup though.
Basketball - the one and only LeBron James. He's already the best player in the league and made history a number of times. Probably the first player since Jordan who has even the smallest most outside shot of challenging Jordan for the title of greatest ever. Everyone knows he can score and rebound and has otherworldly athleticism, but the guy is also an amazing passer, better than most of the PGs in the league. And while he had that moment where he refused to shake hands after the Orlando loss, he's really been very mature about everything and has a lot of loyalty to Akron and Cleveland. Here's to hoping he doesn't leave in 2010(I don't think he will, but you never know).
Basketball: LeBron James because he has the most potential to best the best player in the league because of his athleticism. Kobe performs the best right now, but LBJ can physically do things that Kobe can't.
LeBron is already better than Kobe and in fact the best overall player in the NBA right now. Kobe has a much better team around him(Pau Gasol = All-NBA, Mo Williams = All-star reserve) and is still one of the best players in the game, which is why the Lakers won the title, but Kobe's insane skill with his jump shot and general beautiful textbook basketball form are really the only things he has on LeBron at this point. LeBron is a better passer, driver, rebounder, shotblocker, and didn't spend years pissing off teammates. Kobe is great and has proven to be a winner, but I think LeBron will win some titles as well before his career is over, and right now, LeBron is a better basketball player than Kobe. The stats back it up too.