Rex are worth taming but tek rex and alpha/enraged variants give also legit loot and tons of exp. The best way to farm alpha rex on single player is to fly to carnivore Island and kill a couple of rex until you have the chance of fighting a alpha variant. There's another way, fight on extinction on east,where wyverns and enraged rex are wandering around or even better,fighting tons of 500+ enraged rex and creatures on the legendary OSD(Use the strongest creatures u have in ur arsenal there)