I love the fried Mussels at Ruen Pair, and sometimes I just have to pass up the Pad See Ewe at Samaluang, or the Seafood-chili-garlic-with thai basil at Palms, or the whole fish at Kruang Tedd, for the fried mussels (and the best sauted morning glory) at RP.
After reading about the Thai Wat temple, I finally made it out there this weekend for the first time. Hands down, the fried mussels win the "Best Thai Fried Mussel" award.
The RP mussels are made in a very crunchy batter, reminiscent of fish & chips batter, and you break off large, greasy pieces with no mussels. The ones at the temple have a batter that is more bread-like, more flavorful, less greasy, with crunchy bits mixed in, so that you get all three (crunchy/breading/mussels) in every bite. Pour the sweet sauce over the mixture and it is fantastic!