Favorite equals easiest for me here (since I like those enemies which I have few trouble with), so that list would probably look like this:
Gym Leader: Surge. I never had any problems against him, only with his stupid gym maze. But who doesn't?
Elite 4 member: Bruno. Except for the Steelixes in FRLG, he has never been much of a problem for me. Sidney would be another option for this, if he just wouldn't have that annoying Mightyena with Intimidate.
Champion: Lance. With Ice and Thunder Punch being cheap TMs availiable to many Pokemon, it's (well, most of the time) not too hard to beat a Flying-type (you can't deny it, he always had more Flying types than Dragons, except when counting the second round in HGSS) Champion.