Conan's opinion of Kaito is an odd mix of complete annoyance at his personality and trolling, annoyance at his being the only criminal that keeps getting away, irritation at his chosen career path, and respect. Conan respects Kaito more than he does a lot of detectives. (The incompetent ones, of course).

And as much as Kaito irritates Conan, and as much as he would jump at the chance to get him arrested (at a heist after finally outwitting him), Conan also trusts Kaito to be an ally when people are in danger.

If they weren't on opposite sides of the law they'd probably be friends.