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.