Characters from different fictional universes who would be instant best buddies.

Miles Vorkosigen and Tyrion Lannister.

Or would they instinctively loathe each other?

Sam Malone and Barney Stinson. They’re both babe hounds but they’re working different cities. I was thinking of Barney and Joey Tribbiani, but with both located in NYC they’d actually be rivals.

C-3PO and Marvin the Paranoid Android.

Robbie the Robot from Forbidden Planet and The Robot from Lost in Space.

Add Bob “Bulldog” Briscoe in, and you’ve got a fun babe hound quartet.

Sheriff Andy Taylor and Dr. Leonard McCoy.

Xena and Red Sonja

I think Goku and Thor would be even better friends, and would get a huge kick out of sparring to improve themselves.

I suppose it depends on what Gecko was wearing, what his business card looked like and if he had any videotapes to return.

Doc Savage and the Gray Lensman.
The Shadow and Rorschach.
And Colibri, I remember the crossover Robbie did on Lost in Space. Not the character, he had a different persona, but the same robot.

True, but there is a never ending online nerd debate about who would win in a fight between Goku and Superman but in reality they would never fight because they are both really nice dudes who want to help mankind.

This.

Tyrion = Miles - (Vorkosigan Family) + (Lannister Family)

Lara Croft and Daniel Jackson

Han Solo and Mal Reynolds.

James T. Kirk and Horatio Hornblower

Jean Valjean and Richard Trimble.
Inspector Javert and Lieutenant Gerard.

Jane Cobb and Canderous Ordo.

Tyrion Lanister and Varric Tethras

The irresistible depression meets the unmovable chipperness.

C-3PO can be pretty paranoid himself.

R2-D2 would probably be a BFF with Robbie and The Robot.

Gort and the Iron Giant.

OK on R2-D2, Robbie and The Robot as a threesome, but Gort and the Iron Giant? Gort seemed completely sanguine about being a gun, in fact, it seemed to be his raison d’etre. The Iron Giant would make a total pain in the ass of himself with pacifist speeches and pamphlets. Gort would vaporize The Robot out of sheer annoyance. Superman, indeed. Superhot cloud of glowing plasma is more like it.

Spock and Dr. Lazarus.

Indiana Jones and Lara Croft.

Ellen Ripley and Tasha Yar.

Peter Quincy Taggart, Zapp Brannigan and James T. Kirk.

Gandalf and Dumbledore.

Too late to edit, but of course I meant Gort would vaporize the Iron Giant, not The Robot.