For our purposes here, I will define "modern era" as being from the '70's or more recent. (Most of us aren't old enough to remember anything from the NFL in the 60's.)
I will start it off with the best dream team of LBs, assuming all players were in their primes, for discussion purposes here.
I will also assume we are talking about a 3-4 D (even though the 70's Steelers were running a 4-3 for much of that run).
Even though he doesn't get the recognition of many LBs from the Steelers, I absolutely have to start by putting Harrison on this list. In his prime, there simply was no better LB in the history of the team.
He hits harder than anyone else, he rushes the passer better than anyone else, and he his the biggest penchant for the big play, more than anyone (see 100 yard TD on a pick in the Super Bowl. It gets no bigger than that.)
So Harrison is the one OLB. The other is Greg Lloyd. There have been some OLBs who may be better pass rushers, but no one was as complete as Lloyd. I can't imagine a team with #92 and #95 at OLB. The thought is insane.
On the inside I will go old school with Jack Lambert. His intensity, hitting ability and range was koo koo. He could have played in any era.
Last spot I will give to Kirkland, assuming we are talking about the player before he ate himself out of the league. His combination of size and speed is still unheard of.
Harrison..............Lambert...Kirkland.........L loyd.
It doesn't get any more bad assed than that.
Honorable mentions: Greene, Ham, Porter, Brown, Merriweather.