Considering my opinion is there are so many good defensive lines, it's hard to name what would be the top 5. I will try to name 5 but no promises.
The only one I can say for certain off the top of my head is the Jaguars. Not because their defense is, overall, on paper any better than last season (they've gained some and lost some), but because their defense will "improve" because of their offense keeping their defense off the field.
Even if Nick Foles isn't what they were hoping for, that isn't really a requirement for him to be much better at the quarterback position than what they had a season ago. They also added Jawaan Taylor (considered a steal and where they drafted him and one of the best offensive tackles in the draft), meaning the 2017 version of Fournette is likely coming back. Their d-line has a strong rotation to begin with, but I'm sure they will make life miserable for opposing QBs for the 100% duration of each game.
I'm going to try to finish this now. But there's a strong possibility I might cheat on this one too, and end up having more than 5.
2) The 49ers. For the purpose of this I said only the defensive line, not the defense as a whole.
Everything behind the 49ers defense line could be considered mediocre at best, but their defensive line, when the starters are healthy, is arguably the best in the NFL. The only difficult part is "when healthy". Dee Ford and Nick Bosa already don't have great reputations for staying healthy. Adding them to this list assumes they won't be injured for all or most of the season.
3) Cowboys. Their defensive line is stacked, their linebackers are stacked, their secondary is meh, but you can't have everything. But again, this is only about d-lines anyway. They have good starters and a strong rotation to last all 4 quarters.
4) Bears. I wanted to get cute and creative with this, but there's no denying the obvious. Aaron Donald is better than Khalil Mack but the difference is Mack has a lot of legitimate help outside of himself on the line.
5) Steelers. It was already a thoroughly solid unit, but Devin Bush gives them one of the best front 7's in all of football. The Steelers tied Kansas city last year for having the most sacks. Steelers offense was above average (as usual) but not unbelievable. What Kansas City did last year almost certainly won't be replicated two years in a row, and I don't think any team will in the immediate future. As there will be no one else with this many sack opportunities because of their offense, I am sure the Steelers will easily have the most sacks in 2019.
6) Bills. They already had the best pass defense in the NFL last year, so their pass rush is already fine. But they were ranked 16, middle of the pack, with run defense. Then they drafted Ed Oliver. People have compared him to Aaron Donald, but he doesn't have even be that to take this D-line to another level.
7) Panthers. They'll have far and away the best defensive line in their division. They will have absolutely no problem whatsoever getting to the enemy QB, even for all 4 quarters of the game.
8) Packers. Even if Rashan Gary isn't what they were hoping for, this is a thoroughly solid unit. But if he is, they will be contenders to have the best d-line
period.