I have 06 and love it, but I am looking to upgrade anyways. I've seen that Tiger 07 got better reviews than 08. I can get either one used for less than $30 US at GameStop (EB Games), but it looks like I've been leaning toward 07. TW 07 has more courses than 06.
06 = good, cheap used, limited courses, no longer any DLC
07 = good reviews, more courses, fantasy courses, cheap used, no longer any DLC
08 = bad reviews compared to 06 & 07, under $30 used, still has DLC
09 = better reviews than 08, probably $ 55 used - if you can find it yet