But isn't his stout run D, sideline-to-sideline ability (perhaps synonymous but I digress), and intelligence allow a defense to run a 2-LB system (e.g. 4-2-5) pretty rare? Or are plenty of defenses using that formation without issue? i.e. Are there numerous ILB types who can fill this role?
There's kind of a lot to unpack here. 4-2-5 is probably the most common defensive set in the league, so I guess the answer to your question is yes, defenses are using that formation without issue and many ILBs can and do play in that system.
That doesn't mean Hightower isn't special; he's got the ability to play off-the-ball effectively and blitz from off-ball as well as play on the line and do DE-type things and hold up in pass rush and against the run. That helps a Patriots defensive front that doesn't have great individual pass rushers. He had 5.5 sacks and 13 QB hits last year.
I would not describe him as a sideline-to-sideline type linebacker. His tackle production is pedestrian, especially compared to the names on the list. Hightower's career high is 55 solo tackles; that would represent a career-low for most of the LB who made the 100. As one point of comparison, Fred Warner had 89 solo tackles last year; High had 47.
He's not the kind of athlete you want running with the better TEs and RBs in the league, which sets him apart from the guys here. He does not really post eye-popping playmaking statistics (one INT and 2 FF in his career, though of course he added a very notable FF in the playoffs). In the modern NFL where most teams are running 2 LB sets and asking their 230-lb LB to run and chase, he's not that guy. He does do a lot that the Patriots value highly and that they get a lot of value out of. I think he's a hard guy to rank in an exercise like this.