Liu Kang vs Jin Kazama: A tough question; while Liu Kang has certain killing arts up his sleeve, Jin Kazama seems to be a superior martial artist and has his 'Devil Jin' form. I'd say Jin.
Scorpion vs Kazuya Kazama: Scorpion, hands down. Kazuya has NOTHING to take down a specter.
Solid Snake vs Captain America: Cap. overwhelms in strength and speed (and maybe intelligence), Snake overwhelms in skill. If played stealthily, Snake will honk around with Cap. until he's taken down, but as soon as he's spotted, Snake is as good as gone.
Trunks vs Link: Seriously? All Link has over Trunks is an indestructible shield and legendary sword; Trunks has VASTLY superior strength, speed, and better ki-control.
Akuma vs Nightmare: Akuma has a way of taking down Nightmare's physical form, but no way to destroy the sword. However, he is intelligent enough to avoid using said sword. Akuma, hands down.