What do you guys think is the best overall mouthpiece to use?

A custom-made one from your dentist.

Short of that, I'm a fan of the Brainpad design: Everlast, Shock Doctor, and probably a few other companies make a one-piece upper and lower with breathing holes.

The most common cause of concussion and KO in amateur boxing is a straight punch to the chin, which drives the lower jaw against the base of the skull. The one-piece upper and lower mouthpieces hold your lower jaw forward, which minimizes this impact.

Hey Fraidycat, is it hard to breathe through those holes?

look up Jawz, Its a guy from my area supplying to all the local fighters and UFC guys..

No joke, they will cut down on headaches etc...ALOT

i read good things on jawzz.

i'm getting one from smartguards right now.

I use this one and it's worked great (brain pad pro plus I think)
http://www.comparestoreprices.co.uk/images/br/brain-pad-pro+plus-black.jpg

i got a shock doctor. It was ok, but rubbed on my gums for a few weeks. Never had an issue with it.