Well, if used in a sun team, the Sunkern with Chlorophyll would be the best one to evolve.
In a double battle, while Solar Power is useful as well, it decreases its Hp yet increases its SAtk.
And Early Bird is good with a rest combo, which it can learn, is another option. It can also work.
So all three abilities are good, but in a Sun Team, Chlorophyll, which raises its Spd, is the better ability.