Don't know if they're the best, but when I think of eyecatches I think of Cowboy Bebop. I also like the ones in Patlabor. Shout out to FMA Brotherhood with the guy saying "FULL METAL ALCHEMIST" really weirdly.
Lycoris Recoil is my vote as well. As an aside, I recently started watching Zombie Land Saga and learned that the eyecatches of the girls posing with things are actually products made in Saga prefecture. Anyway, LycoReco's eye catches are great.
KonoSuba is worth mentioning. The "KonoSuba" voice line always has something to do with what the characters were doing/feeling right before the break. Like if Aqua did something cheeky the story, she would say "KonoSuba" in a cheeky voice.
I liked SDF Macross/Robotech "Macross Saga" and Heaven's Lost Property eyecatches. Both were short and thematically appropriate to each respective show, and had a catchy jingle with both.
Neon Genesis Evangelion also.
A Place Further Than The Universe is one of the most visually stunning shows for me. Its backgrounds are amazing. I should rewatch it soon.
My rice had the various ones from Haikyuu as rotating background on her PC. The shear number of them is kind of crazy.
The Digimon 01 eyecatches are pure nostalgia for me, they're the only ones I really remember too well. It's probably because of the catchy music.