Guts is WAY better. With the Flame Orb, his attack rises from a puny max 150 to a monstrous 225. Besides, with a movepool of Brave Bird, Steel Wing, Rage, and Roost (my favorite set. Rage, then Roost), it can hit all ghost types anyway. Adamant is probably the best nature.
To Pokemaniak: I actually did. And DynamicPunch, too! Isn't that AWESOME?
To Quagsireking: Adamant is definitely the best nature for a Crobat, as its attack is only 90, and its speed is 130. However... Golbat is actually very competent with the Eviolite. If it's Modest, it can use Tailwind, Heat Wave, Air Slash / HP Dragon, and heal itself with Giga Drain (or roost, but that makes it vulverable to super-effective ground moves).