All of your options are fine, except for "I want a pizza." In English, this is considered blunt, bordering on rude. I could say "I want a pizza (I'm in the mood for pizza) to my friend, but saying it to a waiter would be bad form.

From your options...I'd like a pizza is best. We normally use the future + the when we order food. " I'll have the four cheese pizza. I'll have the steak with the baked potato, I'll have the burrito con carne. " If you want to be very informal, you can say "Can I get the #22? The pepperoni pizza with extra cheese."

Personally, I'd say I'll have a pizza OR I'd like a pizza.

I would probably say "Could I have a pizza, please?" But any of your last three are fine, providing you remember the 'please'.

I would like to have a pizza or can i have a pizza!