I'd have to say it's a close race between a druid, a hunter and a rogue. The druid because of the broad skillset, the... swiss army knife of classes as it were, as in self-contained tank and healer on the fly.
A hunter because of the ranged factor, camouflage, feign death and the luxury of having a pet(s) as cannon fodder. The hunter seems to me to be the most forgiving class for botting, meaning if something doesn't work as intended, there are many ways to exit stage right as it were.
And a rogue, because of the stealth/vanish factor and combat momentum, a properly configured rogue can perform quests and gathering with an epic degree of precision. Not to mention the ability to regen in combat (though it would be nice if the bot used the remaining action points after combat and dumped them into rogue regen as well).