I'm a numbers guy so after getting the chance to look at all the different player's stats here's what I've come up with. I basically looked at the top 2 seasons each player had but gave additional weight to the best season each player had.
1. Luke Staley
2. Harvey Unga
3. Curtis Brown
4. Jamal Willis
5. Ronney Jenkins
6. Lakei Heimuli
7. Casey Tiumalu
8. Eric Lane
9. Jamaal Williams
10. Brian McKenzie
11. Hema Heimuli
12. Peter Tuipulotu
In addition to that list, here are the top 10 individual seasons by BYU RBs by my estimation in the modern era (Lavell forward):
1 - Luke Staley, 2001
2 - Harvey Unga, 2007
3 - Curtis Brown, 2005
4 - Ronney Jenkins, 1998
5 - Jamal Willis, 1994
6 - Harvey Unga, 2008
7 - Curtis Brown, 2006
8 - Luke Staley, 1999
9 - Casey Tiumalu, 1983
10 - Lakei Heimuli, 1985