Depends on your tastes, and the purpose I imagine. For my Better than Wolves game, I use a texture back called Derivation, as the dark medieval flavour works really well with theme of that mod. The villagers are ugly though, so I swapped them out with villagers from another texture pack.
For the Xenoflux server I'm using a texture pack called BDCraft -otherwise known as Sphax. In contrast to my choice for BTW, this one is full of colour, has a slightly cartoonish feel, and very bright overall.