Gegard IMHO
His UFC run was admittedly not great for his true skill level but there's no doubt he could have been champ with one or two slightly different matchup
With the way Mousasi got dominated by Machida and Jacare I feel like he was just never at that championship level and his highest level win (Weidman) was a clearly out of prime Weidman. Mousasi reminds me of Cerrone where he was a great and elite fighter but not quite championship material with a championship opponent level win.
Where as guys like Overeem (less proof due to PEDS), Joe-B, Faber were without a doubt able to beat every single fighter outside the champ at their peaks and even gave the champions hell in their title shots.
Gegard is overrated, I think people just like saying the word Mousasi.
Mousasi is one of the most wildly overrated fighters in history and one of the greatest examples of the "you don't know him but trust me, he's good" indie darlings. I am a fan of the guy too but come one... there's a decent amount of people who rank him in the top 3 MWs of all time ffs and that is just ludicrous.
When Anderson looked undefeatable during his reign, absolutely any MW fighter outside the UFC immediately got hyped to the moon as some sort of "pending threat" despite the fact that they were all just feasting on cans for years. Denis Kang, Hector Lombard, Alexander Shlemenko, and Mousasi were all GOAT candidates apparently

despite completely getting exposed when their level of competition went up.
Prime Shlemenko lost to an ancient Tito Ortiz by submission FFS. Mousasi definitely faired the best out of all of these guys but he was still an extremely flawed fighter with a clear ceiling.
People love "foreign underground" fighters and they love strikers with "an active guard" because wrestlers were so dominant in mma (back when the sport actually paid well enough to attract them) that people rooted for alternatives but jesus ANY competent wrestler with submission defense absolutely handled Mousasi even in his prime. He got taken down repeatedly in his prime by non-wrestler and UFC castoff Keith Jardine (who has one of the worst chins in history) and couldn't even put him away. Case closed.
As we've seen with Khabib, people will completely gloss over the actual quality of competition someone faced if they have a gaudy record and interesting persona to get behind but unfortunately, Mousasi falls into this category of overhyped record-padder. I always enjoyed watching the guy fight and he as an individual is very humble but his fans are typically obnoxious/delusional. Objectively, his resume is not top 5 MW material (belt or not, it's a prop) and he is far from the greatest fighter to not win the UFC belt.