There are really 2 main players that have a lot of user data for trainers. You should pick one or the other - Kurt Kinetic or Cycleops Fluid2 (or higher).
There are other good ones, but these have the biggest userbase, most familiarity, and most reliability because of it. They also will likely have highest resale value in case you decide it's not for you after a year or two. They hold their value extremely well - see how high they go for on Ebay.