I have wooden rings, plastic rings, aluminum rings, solid steel rings, hollow stainless rings and even a set of rings in the shape of STARS... but when I work, I use 10-inch Owen hollow stainless rings. They have been in use for maybe 30 years and look like new. I have a set of 12-inchers too, but have never used them. I used to use a 16-inch Koran 3-ring set too, but they are in storage.
I have the locking key, but find for a commercial comedy routine the locking key gets in the way.
Additionally, for special occasions, I just got a set of Porper's SPINNING RINGS... with a fantastick locking key... in this context it works like a charm.
Chris Capehart suggests using cheap rings as their flexing helps with some of his moves.
It almost doesn't matter (to the audience) what you use.