

To no one's surprise, Doctor Doom is one of Captain America's best partners. Cap makes excellent use of Doctor Doom's capacity as a damage engine on 2nd, and with Hidden Missiles assist, Cap can act as his own damage engine by looping his incredibly damaging Hyper Charging Star. However, Plasma Beam and Molecular Shield are both also incredibly useful in neutral and in combos. With Doctor Doom 2nd, Cap gains access to incredibly damaging TACs that can win you a game. All in all, you can't go wrong picking Doom to slot behind Cap, on 2nd or 3rd.
Likewise, Vergil's appearance on this list should not surprise you. The Son of Sparda is an incredible partner for Captain America, especially when placed directly behind him on Second position. Captain America can DHC directly from Charging Star into Spiral Swords for incredibly damaging extensions that secure kills which he might otherwise struggle with. Furthermore, Captain America's Shield Slash assist allows Vergil to pick up off High Time, making this a uniquely utilitarian pairing. Rapid Slash is an incredible assist in neutral for Captain America, as he can throw out a charging star to catch the opponent pressing and, if he is blocked or whiffs, make it safe with Rapid Slash. S-tier pairing.
Spencer, like other damage engines, is quite useful for Captain America. Cap doesn't use bounces, so Spencer gets full looped Maneuvers after Hyper Charging Star. This combined with a good beam on third and Spencer incomings can make this a truly nasty pair. Additionally, cartwheel + slant shot is a very potent neutral and incoming tool. Spencer compliments Cap very well, and will usually be placed directly behind him on the Second position.

Iron Mans Unibeam is, arguably, the single best assist in the game for Captain America. However, it is attached to the albatross that is Iron Man. Struggle Doom can do everything that the Lord of Latveria can do, but he has to work a lot harder to get there. Still, if you really like the character, he's an excellent choice to back up the Captain, especially when paired with the likes of Doom's hidden missiles or Sentinel's drones.
Cap/Sentinel is a time-tested pairing with a surprisingly high ceiling. Cap makes great use of drones in neutral between cartwheel and anti-ground kick. He gets unique extensions, such as looping charging stars with proper spacing. Plasma Storm is a very damaging DHC and Hyper Sentinel Force is a safe one, making Sentinel a capable 2nd or 3rd behind Cap.

Rocket Raccoon is arguably the third best support character in the game. This is just as true for Captain America, who makes great use of his Spitfire or Log Trap assists in neutral and in combos. Rocket 2nd gives Cap a damaging DHC in Rock n Roll and a safe one in Mad Hopper. Especially when paired with Doctor Strange's Bolts of Balthakk, this is a very tricksy and powerful pair.

Bolts of Balthakk has a case for Captain America's single best assist in neutral and combos. It allows for unique extensions that build oodles of meter and for uniquely powerful cartwheel mixups. Doctor Strange makes a powerful Anchor behind Captain America and strong mid such as Doctor Doom, Rocket Racoon, or Vergil. Having Doctor Strange on third means that Captain America can take advantage of Spell of Vishanti THC. The knockdown off of Hyper Charging Star lasts long enough to link an OTG THC afterward - if you're willing to spend 4 bars.
Daggers is another of Cap's best assists in neutral, and Arthur backs up the captain very well. Goddess Bracelet is a highly-damaging DHC and King's Armor is a safe one. Cap backed up by gold daggers is truly belligerent, though he can do work even with the unempowered ones. Arthur thus makes an excellent 2nd or 3rd behind Cap when paired with the likes of Doctor Doom or Vergil.
Last but not least, Strider Hiryu is a very powerful partner for Cap, with his strong OTG game and lack of anti-air options. Captain America backed by Vergil and Strider in particular is a very nasty team with surprisingly strong anti-meta capabilities. Vajra combined with Cartwheel is very strong, and Cap is one of the best characters in the game at converting off of it. Great pair.