Obviously rankings are subjective. For my own personal rankings I am doing records in the last three years with quality wins and losses taken into account, as well who I think would win head to head.

LW
1. BJ Penn: quality wins over Sherk, Stevenson, Pulver with a bad loss to GSP.
2. Sean Sherk: wins- Florian, Diaz, Franca, Griffin; losses- Penn
3. Kenny Florian: wins- Huerta, Stevenson, Thomas, Lauzon; losses- sherk
4. Diego Sanchez: wins- stevenson, riggs, parisyan; losses- fitch, koscheck. Sanchez is lower than florian because he has only one fight at 155, while florian has quite a few quality wins at the weight.
5. Maynard: wins- edgar, miller, clementi; losses- none
6. Frank Edgar: wins- griffin, franca, fisher; losses- maynard
7. Tyson Griffin: wins- guida, aurelio, tibau, tavares; losses- sherk, edgar
8. Joe stevenson: wins- pellegrino, tibau, edwards, guillard; losses- penn, florian, sanchez.
9. clay guida: wins- aurelio, diaz, danzig; losses- huerta, thomas, griffin.
10. nate diaz: wins- pellegrino, neer, robinson; losses- clay guida

to be continued..