I like it with two n's.
Denis looks too much like....... Well...... It could be my rather immature mind kicking in here but I can't help but think of him being teased as "DEE-nis the PEE..." Hopefully you catch my drift.
I think with
Dennis it's easier to see the proper pronunciation. Also, like Array, I like the idea of
Denny. But that might be because I'm a
Grey's Anatomy fan!
I've known a number of people named
Dennis... I guess it's rather popular around here, or was at one point. The most notable was a boy in my grade, who was probably at least a foot shorter than anyone else in the class. I was his friend, being a shrimp myself, and we liked to collect rocks at recess. But he grew up to be pretty odd. So that's my impression of the name! A shrimpy rock-collector who never got over his awkward phase.