Yeah I guess there are 2 periods to consider - Dynamic and Select. The Dynamic folders are definitely the expensive ones. In terms of hardest to collect I personally rank them as...
Select
1. 2001 Impact
2. 2000 NRL
3. 2005 Tradition/2006 Accolade
Most of the select folders post 2002 are a pretty easy pickup for about $30.
Dynamic
1. 1997 Fatty Album (the unsigned version)
2. 1991 Stimorol (very hard to find without the plastic bubbled on the front)
3. 1996 Booster set (most of the 3 1996 folders are hard to find - most collectors had walked away from the hobby)