Cap? Iron Man?...
I'd say he is closest to Captain America but is also friends with Tony Stark; the difference is that Stark has probably annoyed/hurt Thor more with his less than favorable (but realistic) actions over the past few years while Cap has been largely a hero in the Asgardian mould (valiant, morally upright, strikes awesome poses). The dwarves and other craftsmen would understand and like Stark more, one would think.
an honorable mention would be Spider-Man; the most human and neurotic superhero can banter it out with the best.
Beta Ray Bill counts as an Asgardian I suppose, so his godly best friend is most likely Hercules- who else would Thor let play pranks on him (sans Loki) and laugh it off?