Isekai Quartet is one of the anime airing this season. It's not finished yet, but I'd like to give my thoughts about it here.
Overlord is by far my favorite out of the four. I love the characters; they bounce off each other in a way that I find hilarious, especially in the spin-offs like A La Carte and Ple Ple Pleiades. A common complaint I see about Overlord is that the guardians have no/the same personality because they're all loyal to Ainz. This might seem true to an outsider, but as someone who's a bit closer with the series, I feel that the characters are distinct and interesting.
Overlord has, in my opinion, carried Isekai Quartet so far. Ainz has done the most legwork interacting with the other characters, which is the point of a crossover show. He even ends an episode in the most comfy way possible, but I'll let you experience that yourself if you haven't seen Isekai Quartet yet. Ainz should also be the most relatable for the average viewer, even if he's a skeleton lich Sorcerer King.
For some reason, r/anime and /a/ just love KonoSuba. While I actually used to like KonoSuba a lot, constantly hearing exaggerations about how hilarious or based it is has turned me off of the show. (So much, in fact, that it has made me lower my score of it on Anilist.) My biggest issue with KonoSuba in Isekai Quartet is the repetitiveness of their gag humor. This is especially true of Darkness, who pretty much has one joke in both Isekai Quartet and the KonoSuba anime. I still do like Darkness in KonoSuba, but in Isekai Quartet, she's just outright annoying. Megumin also hasn't done much in Isekai Quartet yet either, so sorry if you're a Megumin fan.
Honestly, watching Isekai Quartet made me realize why Aqua is so popular, even if Megumin is the "canon" ship with Kazuma. Aqua and Kazuma are hilarious. Their jokes felt the least repetitive, and in Episode 8, Aqua has a hilarious interaction with Albedo and Shalltear that just made me laugh because of the connection that was being made across the two anime. I still find /a/'s constant praise of KonoSuba to be worrying, but I shouldn't let that stop me from enjoying an otherwise good anime.
Oof, Re:Zero has pretty much always been polarizing. A lot of people (including the characters IN the show) find Subaru obnoxious, but I actually like him. He has a wholesome hikki-esque sense of humor that a lot of people think about but don't say. Also, the Re:Zero anime (out of these four anime) is the most compelling to watch on an episode-to-episode basis. I still remember sitting on the edge of my seat watching Re:Zero for the first time and having the ending theme send chills down my spine.
As for Re:Zero's role in Isekai Quartet, I personally find it to be the weakest link. Roswaal's voice is grating, Emilia & Beatrice have minor roles, and Rem's character is just "I love Subaru" now. However, I find Ram's smirk to be hilarious, and I still find Subaru's antics to be endearing, even if annoying.
Youjo Senki is by far the most unique isekai on this list. It's ugly, unfamiliar, and features a completely unrelatable protagonist. Because of that, it is the least popular of the four anime, but I would say it has the most purpose to it. Overlord (in the anime so far) has mostly been a power fantasy. KonoSuba is a comedy. Re:Zero has had more meat to it, bringing out the most extremes of its characters to grip the audience, but the ending of season 1 made it feel like the show had no purpose. On the other hand, Youjo Senki has a goal and path to the story with unique themes, settings and power system that're interesting to learn about. However, because of Youjo Senki's heavy focus on these topics, the characters are sparse and extremely boring.
A comedy like Isekai Quartet is carried by the interactions between characters. Although Youjo Senki brings a squadron of people, it really has two characters, (Tanya and Viktoriya) and Viktoriya has yet to do anything interesting. But, Tanya has had a golden moment with Ainz where they enjoy themselves, which is honestly a breath of fresh air for me and the both of them. Both of them live stressful lives, feeling alone and disconnected with the people around them due to their leadership status, so it was actually nice to see them become good friends that can relate to one another. Tanya also gets along with Ainz's close "friend" Demiurge, which also brought a smile to my face.
Summary - Is it worth watching?
Honestly? Is this show worth watching? If you've already seen all four of these shows, then sure, it's a nice filler anime. But if you don't like one of the anime on the list, (for instance, you think Overlord's characters are boring or you don't appreciate KonoSuba's humor) then it may not be for you. Either way, it's not a huge waste of time, as it's only going to be 12 episodes that are around 12 minutes long each. So far, I would rate the show somewhere between a 5 and a 7, but it's not finished yet so it's hard to say definitively.