As was mentioned 1 tooth on the front + about 3 tooth on the rear. Front sprockets are cheap and easy to change. I would experiment with fronts and see what you like. with the gearing calculator on e2s or simple devision you can figure out the ratio-- than you can achieve the same ratio by going to differeent combinations of front and rear spreockets.
A 12 tooth is very small. It is a tight bend and puts more strain on the chain than a 13, or 14 tooth. I try to find the ratio that performs best--than I run a 14 tooth front and adjust the rear to get the same ratio. I usually gear down for dunes so I can grab the next gear, and gear up for fast desert riding. Its all about your preference and what works best for you. Dont be afraid to experiment.
To your origional question on best sprockets. Go with steel vs aluminum unless you have money to burn. You will NEVER notice the weight difference and steel lasts so much longer. I get all mine from rockey mountain atv.com and get the Tusk brand steel sprockets. They are very inexpensive, good quality and last.
How do you trail ride on 18"s and not drab your rear end. I run 22's and still pound my skid plate every now and then.