Agreed on the Tokina. I've seen the most people using this, and I like it too. If you have a higher budget, you could look at Canons L series, like the 8-15mm fisheye or 14mm wide angle but they are $1,500 and $2,400. So its hard to beat Tokina's wide angle for only $500.
Look it up on Amazon or B&H and check out the customer images - they're all pretty sharp.
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