One aspect of answering this question is to ask another obvious one: "which of these teams would win a game if they played each other?" For example, would 1971 Nebraska beat 2019 LSU in a head-to-head game?
Of course they wouldn't; it wouldn't even be close. Tom Osborne - who was the offensive coordinator of the 1971 Nebraska team - said prior to the 1984 Orange Bowl that his 1983 team would blow that one off the field offensively, it wouldn't even be close, if anyone should know, it's the guy involved with both teams.
Would 2019 LSU beat 2018 Clemson?
Well, they manhandled 2019 Clemson, didn't they?
Granted, Clemson lost 3 defensive linemen to the first round of the NFL draft. But in light of the fact that 2018 Clemson gave up 12.9 ppg and 2019 Clemson gave up 11.5 ppg (pre-championship game stats) and lost a linebacker and a corner in the first round, I'm not sure how the argument holds that 2018 Clemson's D was better than 2019, at least not that much better. The 2019 Clemson defense gave up fewer points per game (until you add the blowout loss in the final), and the stats were so close it wasn't funny.
DEFENSIVE STATS PER GAME (includes title game)
2018 Total Defense Yards: 291.5
2019 Total Yards: 288.5
2018 Pass Yards: 189.6
2019 Pass Yards: 172.3
2018 Rush Yards: 101.9
2019 Rush Yards: 116.3
18 Clemson's D was better against the run but not as good against the pass, although this includes Burrow throwing for 463 yards against them and Tua "only" 295. In other words, the 19 Clemson pass defense was better INCLUDING the Burrow game than was 2018. So I'm not sure how to make the argument 2019 LSU would not have blown 2018 Clemson off the field with the same ease. Neither year did Clemson have an even remotely challenging schedule (one ranked ACC opponent each year plus the Aggies, who were better in 18 than in 19 and the game was at Kyle Field).
And bear in mind, I haven't even touched on the offense yet.
Trevor Lawrence:
2018 - 259 for 397 (65.2%), 3280, 30 TD, 4 INT
2019 - 268 for 407 (65.8%), 3665, 36 TD, 8 INT
Of course, the immediate objection says, "But Trevor wasn't the starter early in 2018, and he went out of the Syracuse game with a concussion." True. But in light of the fact he only had TEN MORE ATTEMPTS and completed NINE of them and six were TDs, that kind of pushes that objection aside. And he had more yards per attempt, yards per completion, and yards per game, so I don't know how that can be turned into "but he got hurt in one of the games and wasn't the starter" since the adjusted stats still show he was better. The Clemson offense in 2019 had 0.9 ypg more in total offense, an insignificant difference that might be chalked up to one easier game.
This is why I'm not sold on 2018 Clemson as even being worthy of being mentioned in the same breath as the others. They, to me, are like the hyperbole that surrounds 1995 Nebraska, who wasn't even favored to win their title game but they moment they blew out the opponent became the "greatest team of all-time."
The FACT is that 2019 LSU blew 2019 Clemson off the field, and an argument can be made that the 19 Clemson team was better than the 18 Clemson team. Just because one won the title and the other didn't doesn't mean the championship team was actually better.
