Which geranium has the largest bloom?

Definitely pick an annual geranium over a perennial geranium for large flowers. Remember, plant health is an important factor with growing large flowers. In your zone 8 garden, you might want to give them part sun rather than full sun. Watering, deadheading and applying a balanced fertilizer will keep the flowers coming on. As for the largest flowering variety, check in with your local garden for their recommendations.