Mossberg 500
I vote for the 500 for 3 simple reasons:
1. It's the one I own
2. The simplicity of the gun
3. The reliability of the gun
A friend recently asked me to help him with his 500. When I disassembled it, I found that it had been badly neglected (to the point of abuse). Rust, rust, rust...I'm sure this gun lived in a pond for awhile. Anyway, even in that condition, it still fired reliably. For the reasons above, and a fair price, you can't beat it.