great, supreme, true, undisputed
current, defending, reigning
The reigning champion will defend her title tonight.

former, past
undefeated
three-time, etc.
all-around (AmE), national, state, world
junior
Olympic
the Olympic skating champion

boxing, chess, etc.
heavyweight, middleweight, welterweight

become, be crowned, be named
He was crowned champion after his fight in Atlanta.

beat, defeat
decide, determine
The two winners will move onto a one-game final to determine the champion.

defend her/his title

fighter, jockey, sprinter, etc.

Champion is used with these nouns as the object:
cause, interest, right