If sorted by importance/difficulty of position it goes likes this: LOLB most difficult because you're playing the TE and a good team will often ask you to cover that TE 1-1, sometimes without help over the top. Also, most of the dangerous outside runs go to that side so you will need enough speed to catch fast HBs.
MLB comes second. You're stopping the inside gaps on run plays and help on outside runs, if the OLB gets held up by a blocker. Very important is also the defensive captain skill here. There are basically two types of MLBs: slower run stoppers and side to side tacklers that are less effective against bull rushers.
ROLB is the least important on most teams. You can get the best results with someone who is a real pass rush specialist, but those are hard to find. Most teams will therefore sub this LB out during Nickel packages. Apart form pass rushing he's responsible for outside runs and covering any back running a pass route to that side. So speed is his most important asset.
MLB comes second. You're stopping the inside gaps on run plays and help on outside runs, if the OLB gets held up by a blocker. Very important is also the defensive captain skill here. There are basically two types of MLBs: slower run stoppers and side to side tacklers that are less effective against bull rushers.
ROLB is the least important on most teams. You can get the best results with someone who is a real pass rush specialist, but those are hard to find. Most teams will therefore sub this LB out during Nickel packages. Apart form pass rushing he's responsible for outside runs and covering any back running a pass route to that side. So speed is his most important asset.