Which is the best cookware to cook oats?!

Personally, I think it's whichever is the easiest one *to clean* after the oatmeal is cooked and begins to act like glue when drying!

That would mean glass or ceramic first, and after that smooth shiny metal (not dull metal or brushed metal, etc).
Or something that can be thrown away like a paper bowl, or even parchment paper or aluminum foil (depending on which heating equipment you use--stovetop, microwave oven, regular oven, slow cooker, etc).
(Non-stick pans would probably be best re non-sticking, but I try to avoid using non-stick materials as much as possible myself.)

When cooking it in a slow cooker ("CrockPot")--especially steel cut oats and overnight-- you can also put a small ceramic/etc bowl *inside* the slow cooker's crock (then add water around it) so that none of the oatmeal will actually ever get on the slow cooker, and only one (easy-to-clean) bowl has to be used and later washed:
http://annkroeker.com/2008/03/25/overnig…

There are lots of ways to cook oatmeal though, especially if you don't care about the clean-up factor!
http://www.google.com/images?q=how+to+co…
http://www.google.com/search?q=how+to+co…

Frankly, just a good non-stick pan of any kind will be perfect for cooking your oats. Simple and inexpensive. In fact, you can even cook oats in a crock pot, which I just found out.

http://www.bestcrockpotrecipes.net/