What in your opinion.. are some Great "Guitar Chord Workouts"
in songs?

Songs with hard Chord Changes (on guitar)

an obvious choice is Giant Steps

What are some other ones?
Under The Bridge by RHCP has some medium chord changes in the song. Yeah, what's your level so we can know how to better inform you.
Portugal. The Man »–
Been playing for about 19 years..

Looking for "opinions" on the "Harder" chord changes

Jazz guitar players I guess

stuff in the vein of giant steps, maybe joe pass? ect
Try the acoustic breakdown in the middle of Dream On (Aerosmith). NOT the lead part - the rhythm going on behind it has some very cool/difficult chords and rhythms going on.