Good question, but I can most definitely tell you that Blue is the one that has the highest levelled Pokemon, and he gives you the best experience overall. You get to fight him every Sunday night, which isn't so convenient, and because his Pokemon are high level (Lv. 70s) it will not be as easy as sweeping through the other gym leaders.
And just like Draco Arceus said, fighting all of them gets you the most experience!
Also, from personal experience, I wouldn't bother re-matching Whitney too many times, because the Pokemon she has all give low experience. You're better off waiting another time and fighting other gym leaders than her, because its really slow when fighting her. (She also has an annoying Shedinja, which gives the lowest experience!)
When I get to playing my Soul Silver, I will try and rematch all of them and enter information for the total experience given from each gym leader for your reference.
