For the one match GOAT, Hoad and Vines are strong contenders. Vines could extend that into tournaments and head-to-head tours better than Hoad, though.
Actually, Vines had a mediocre tournament record, and his Davis Cup play was abysmal. He won no big tournaments in 1933, although playing well at Wimbledon, where he lost a great final to Crawford. (The equivalent match for Hoad was the 1956 Wimbledon final, where he beat Rosewall, as good a player as Crawford.)
Vines' failure to defend his Wimbledon title was perhaps the biggest failure of his career. His Davis Cup losses to Borotra and von Cramm were equally disappointing. Hoad seemed to rise to the big occasion better than Vines.
Vines had his big moments mostly in the USA, and apart from Wimbledon 1932, he played below his best in Europe, even losing a tour to Perry, a notably inferior player.