This argument has kept going back and forth between which is the goat (greatest of all time) on the Isekai genre. Of course we could go into the whole manga realm which I have not much information, however, on the anime front, we do see a big rivarly between:
Mushoku no Tensei
ReZero
Konosuba
However, because Konosuba is more of a comedy, it's usually taken out of the conversation. However ReZero and MT, have a lot of similarities. A great world building, lot's of mystery and a pretty long story. Here is when the conversation start splitting, manga readers will argue the world of MT is incredibly larger and a have a history way deeper than ReZero. But on the anime realm is ReZero which has the most seasons.
ReZero is sort of the OG, and to me at least. I give the title to ReZero so far. But that's my opinion. We can also compare it on other categories besides longevity and world building and here is the fact of character design, where Subaru (from ReZero) seemed a bit more of a faulty main character. Not too the extreme of Shiniji from Evangelion, but it is faulty. And many characters despise him as weak, specially RAM. On the other side, we have Rudeus which as far as I have seen, he is way more shy and humble. Making even the tsundere girls loose the grip on him.
I would say that Rudeus is more the hero type than ReZero's Subaru, and I think here is where we can see a more distinguish difference as readers might lean on a 'type' that will feel more related. As far as the story, MT has fall on the controversial alignments some characters celebrate as it went from harmless harems to real marriage and descent. Here is where the longevity of the story would also play on many readers being able to manage many taboo-ish situations.
What about the other characters, certainly ReZero have some situations that have caused many of the characters to jump in and out of the story line, which could be taken as continuation missteps.