The road. Beautiful! Sealed all the way to ChiangMai, and most of the time a motorbike/bike lane thrown in. Don't be fooled by where the bike is, it's left side of the road riding in Thailand. (Mr Pumpy crossed the road to take a drink and a breather.)
It's pretty much dead flat all the way from the border, through Bangkok to Lampang, from where there's a few good sized hills to get over just before ChiangMai.