This all depends upon what you want to achieve.
For comfort, we recommend Colours Red or Elite (if you want a sponge rubber underlay), or Dreamwalk or Softwalk (if you want a PU foam underlay). For warmth, stick with Dreamwalk or Softwalk as these have the highest tog.
For the ultimate in durability, go for a crumb rubber underlay such as System 10 or Treadmore.
If you are fitting a wood or laminate floor then Duratex/Multitex is a good choice if the floor is completely dry, or Timbermate Excel or Silentfloor Gold if you need a built in vapour barrier.