It looks like based on this article, leather is still king; however, there maybe other considerations, depending on your lifestyle.

http://www.vfrworld.com/tex_vfr/tech/ridesuit.htm

I use an Aerostich Roadcrafter one piece suit and couldn't be happier with it for my use.

I have a kangaroo hide 1 piece leather suit. It's considerably lighter than cow hide and apparently has much better abrasion resistance. I find it to be comfortable while riding, standing not as much but that's ok I didn't buy it to stand around in it. Obviously it's not as waterproof as synthetic materials but that's what rain suits are for. Crash protection is top notch, trust me I know, and it vents well in the heat. While I'm sure I'd be fine in a synthetic suit, leather gives me peace of mind and at no point does it feel like too much.