ReetsAdeetsI have had some Black Diamond Mercury Mitts for years now that I like a lot and they are still going strong. Very warm and waterproof, but they were a bit spendy...as I said though, they have worked great for years.
These mitts are the best, if you have the cash. Warmest mittens you can buy, very waterproof and they have a finger slot inside for your index finger for added dexterity. The only thing I don't like about them is that they are gauntlet style and I like under the cuff gloves/mitts, so I just end up shoving the gauntlet under anyways which can be difficult sometimes.
I also own a pair of pow gtx mitts and they are pretty nice. http://www.powgloves.com/collections/frontpage/products/royal-gtx-mitt-15
Not as warm as the mercury mitts but they fit under the cuff easier and I've ridden them all day in the rain and they never soaked through.