which MCU Avenger has the best supporting Characters

Either Thor or Black Panther probably. Most of the others only really have one particularly good supporting character. I'd say Cap has two (Bucky and Peggy) but for three movies that's not that impressive. Iron Man miraculously has very few notable supporting characters for his number of appearances honestly.

Black Panther or Iron man.

Thor or Iron man. I really like Happy Hogan but he along PP is the only one consistent supporting character. Thor has Loki, Asgard, doctor Selvig and I don't even wanna mention Ragnarok which sollos all the others. So, yeah, Thor.

Iron Man or Thor

Thor, Loki is the best non main character in the MCU especially if you count in Odin, Valkyrie, and Hulk.

Cap

Black Panther. Every supporting character there mattered, to both T'Challa's story and the overall narrative.

Thor is a close second simply because his supporting cast is hilarious , at least where his last movie is concerned. The cast from 1 and 2 were pretty bland though, except Loki.

Black Panther

Black Panther.

Luis is the best supporting character in any MCU film

Goop, Jon Favreau, Don Cheadle, Rebecca Hall, Sam Rockwell, Jeff Bridges, Scarlett Johansson, Mickey Rourke, Guy Pearce, Ben Kingsley

They put a little more money into casting an Iron man movie for good reason.

Tony.

Thor by far.

Loki solos, add Hulk and its a stomp.