Going by series that are strictly Isekai there have been a few manga adaptations I'm enjoying but to give an honest assessment, I think I'll only recommend ones that I have been following for over 25 chapters.
There's a bunch that only have 13-15 chapters out and quite a few turned to shit after the introduction. GATE and Mushoku Tensei have been consistent but Re:monster, Overlord, The New Gate, Shiro no Koukoku Monogatari, Tate no Yuusha no Nariagari, and World Customize Creator have basically had nothing happen for months. Just going through the motions. Then there's series where nothing happens period like VRMMO, Death March kara Hajimaru Isekai Kyousoukyoku, Ore to Kawazu-san no Isekai Hourouki, Chihoukishi Hans no Junan, and a few others that focus on things like dandelion coffee, cooking; arts and crafts more so than adventuring in a fantasy setting.
My favorites so far are Kumo desu ga, nani ka? and Tensei Shitara Slime Datta Ken but everyone says the power creep in the former ends up being ridiculous which really ruined Re:monster. The fact that the protagonists lack a penis and a harem might help. Re:monster and Mushoku Tensei are so shameless about it, I feel embarrassed for the author. Since Kazuma is scum and everyone acknowledges it, his harem is tolerable on the other hand.
Actually I think I'm on to something. The series where the protagonist isn't a typical Japanese male seem to be the best unless it's lampshaded. The stunted sexuality, the commentary on the genre, the profound ignorance of feudal societies and aristocracy all get tiring very fast. Kazuma is at least aware of all these things and seems to break the mold on them. He's a genuine pervert. He quickly realizes how little romance their is in adventuring in the dark ages and without a hero's blessing he's basically a peasant.
My biggest gripe about Re:Zero is that Subaru isn't maturing. Most of the drama comes from his impulsiveness and inability to learn.