Jetboil for me. I also take a small selection of tea and coffee sachets, and a small bottle of fresh milk (enough for about 4 brews).
The Jetboil isn't cheap, but I use it mountain walking too so it's been a great investment. Packs down really compact and boils really fast .
The best thing is that, unlike a flask, it doesn't start to taste revolting towards the end of the day - every brew is fresh and hot!