I understand and agree with what's mostly been said, but what it all comes down to (to me) is personality.

Which character do we feel more drawn to - and this is a fundamental point regardless of scripts, plots and actors.

I enjoy Monk, but I can't feel as drawn to his character as I do Columbo's. I took to Columbo's character straight from the beginning and welcomed him with open arms whereas Monk's character I feel I hold at arm's length.

Columbo's early scripts are fantastic whereas Monk's are just very good - IMO.

Also, good old Columbo didn't have a sidekick - unless you count his beloved dog ;)