I will have to go with Ann. The chemistry between Ann and Ken is just great. Such chemistry is so rare ... almost real. In Sawan Bieng, I can feel "it". In Oum Ruk, it is unbelievably great - so real!
Although Ken works really well with Janie, I feel that with Janie it is too brother-cum-sister like. Not sure why I get this feeling even though both of them when standing next to each other looks good - in terms of height, etc.
As for Aom, having watched in Ruk Tur Took Wan (one of my favourite lakorns), I do not think the chemistry has that much of spark. The lakorn story is good and the acting is great, making it a very enjoyable lakorn. But chemistry wise, I still feel that it is below par of what I have seen between Ann and Ken.