Team Theory
Captain America is a Point character. He greatly benefits from two strong supports, and has very specific needs to thrive. He requires strong horizontal coverage, strong vertical coverage, and assists that help him convert off charging star. He additionally loves to have a damage engine mid (such as Vergil, Spencer, or MODOK), whom he can DHC to off awkward hits in order to secure kills.
Best Friends
Captain America's best friends in no particular order are:
Doctor Doom
Doctor Doom is probably the single best support for Captain America. The Lord of Latveria's Plasma Beam assist is extremely strong for Cap, and if played mid, doom's TACs and hard tags both benefit Cap greatly. If placed on anchor with a damage engine in the middle, Doom's support can still be extremely strong. Hidden Missiles is arguably Cap's single best assist, allowing him to link Hyper Charging Star into Hyper Charging Star, but it requires another horizontal assist to supplement it to justify running it over Plasma Beam. Molecular Shield is the most niche of the three, but it gives cap easy grab confirms and helps to cover his grounded approaches. All in all, cap/doom is an extremely strong pairing and worth your time.
Doctor Strange
Doctor Strange's Bolts of Balthakk is arguably Cap's single best horizontal assist, allowing him to loop charging stars in the corner with ease and aplomb. Captain America can additionally tag into Doctor Strange for palm loops relatively easily off shield slash from midscreen. Cap can link Hyper Charging Star into team super with Vishanti behind him, allowing him to dump meter for huge damage. All in all, Strange is absolutely one of Cap's best friends.