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.
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.
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.