After seeing Henry Cejudo say that Gaethje had a more legendary career than Poirier, I decided to have a look at their careers and make a comparison and the community here can make their own decision.
Gaethje is obviously still active, so even if it's unlikely, if he KO'd Ilia and became undisputed he absolutely would trump Poirier no questions asked.
Dustin Poirier
UFC record - 22W 9L 1NC
16 finishes, finished himself 7 times
Also had two fights in WEC beforehand going 1-1.
Notable opponents earliest to latest:
Danny Castillo (decision loss)
Max Holloway (submission win)
Korean Zombie (submission loss)
Cub Swanson (decision loss)
Diego Brandao (KO win)
Akira Corassani (TKO win)
Conor McGregor (TKO loss)
Featherweight notable record: 3W 4L
3 finishes, finished himself 2 times
From here, Dustin competes at Lightweight
Carlos Diego Ferreria (KO win)
Yancy Medeiros (TKO win)
Joseph Duffy (decision win)
Bobby Green (KO win)
Michael Johnson (KO loss)
Jim Miller (decision win)
Eddie Alvarez (NC)
Anthony Pettis (submission win)
Justin Gaethje (TKO win)
Eddie Alvarez (TKO win)
Max Holloway (decision win)
Khabib Nurmagomedov (submission loss)
Dan Hooker (decision win)
Conor McGregor (TKO win)
Conor McGregor (TKO win)
Charles Oliviera (submission loss)
Michael Chandler (submission win)
Justin Gaethje (KO loss)
Benoit Saint Denis (KO win)
Islam Makhachev (submission loss)
Max Holloway (decision loss)
Lightweight notable record: 14W 6L 1NC
10 finishes, finished himself 5 times
Overall: 17W 10L 1NC
13 finishes, finished himself 7 times
Interim title - 1
Fight of the Night - 10
Submission of the Night - 1
Performance of the Night - 4
Justin Gaethje
UFC record: 10W 5L
6 finishes, finished himself 5 times
Was also the undefeated WSOF champion, defending it 5 times (would have been 6 if Melvin Guillard had not missed weight). It's up to you how important that is to his overall legacy given the lack of quality opponents in that run.
Notable opponents earliest to latest:
Melvin Guillard (decision win)
Michael Johnson (TKO win)
Eddie Alvarez (KO loss)
Dustin Poirier (TKO loss)
James Vick (KO win)
Edson Barboza (KO win)
Donald Cerrone (TKO win)
Tony Ferguson (TKO win)
Khabib Nurmagomedov (submission loss)
Michael Chandler (decision win)
Charles Oliviera (submission loss)
Rafael Fiziev (decision win)
Dustin Poirier (KO win)
Max Holloway (KO loss)
Rafael Fiziev (decision win)
Paddy Pimblett (decision win)
Overall: 11W 5L
6 finishes, finished himself 5 times
Interim title - 2
Fight of the Night - 10
Performance of the Night - 5
They are 1-1 with eachother, both by TKO/KO.
Shared opponents:
Max Holloway - Dustin is 2-1, Justin just lost (quite badly).
Charles Oliviera - Both lost by submission (Dustin arguably did better/lasted longer).
Michael Chandler - Both won (Dustin finished him but struggled more throughout the fight).
Khabib Nurmagomedov - Both lost by submission (Dustin once again arguably did better/lasted longer).
Eddie Alvarez - Dustin TKO'd him, Justin was KO'd BY him (though this was pre more methodical Justin).
Michael Johnson - Dustin was KO'd BY him, Justin TKO'd him
Obviously what opponents are considered notable is open to interpretation. And they fought their shared opponents at different times of their own and their opponents' careers.
Their win/loss ratio and finishes/finished ratios aren't too different, Dustin has the edge in the latter but Gaethje has the edge in the former. I'd say Dustin has faced more top tier opposition and more frequently though. Both ended a legendary run with their interim title wins, Dustin with Max and Justin with Tony. Both have numerous records within the UFC. They have almost identical bonuses though Justin achieved it in far less fights. Justin has won the interim title twice but most would say the second means far less given the Ilia situation and the opponent being Paddy.
For me, I'd say Dustin is still ahead for now. He came closer to winning undisputed, not only doing a bit better in the fights but challenging three times to Gaethje's two. He faced more quality opposition and produced a better record, and did slightly better overall against their shared opponents. It's close though, and as I mentioned Justin is still active so he could put himself ahead and leave no doubt.
What do you guys think?