AI and KG are the obvious choices... doesn't make them valid, though. Just cause you're from the hood and scream around like a madman doesn't mean you got more heart than the next player, it just means you're more extroverted. If for one at least don't think KG's got more heart than, say, Battier or D-How.
How can you measure heart? We have no idea how much heart these guys have. Im sure there are role player and minimum wage players who have as much heart as guys like KG, AI, Kobe but just dont have the talent to get to that level. Whats heart based on? Playing through injuries, desire to win? Tons of players in the league other than AI do this but just are not superstars so we dont consider them to have as much heart