As the Prequels are never made, and the OP refers to the pre-PT era, so… according to Lucas and Lucasfilm with the Clone Wars ending in 35BBY, Anakin being born in 55BBY, and Anakin becoming Darth Vader in 18BBY:
Thinking aloud it’d have to be someone capable of playing Anakin across the 20-35 year old age range, maybe a little more on the older age scale if the films are showing early events of the Jedi Purge, so 20-40-ish.
Eddie Redmayne for me. Has a Mark Hamill-esque quality to some of his acting too, and has what would be needed to play Anakin/Vader (and similar features to Mark).
As the Prequels were never made, and I guess the Sequels were also never made, then Adam Driver would have all the qualities needed for a good Anakin (plus he’s basically already played a version of ‘Anakin in reverse’ in the ST films).
Heath Ledger would have made a good Anakin years ago. RIP.
I’d opt for someone like a younger Adrian Paul to have played Anakin back then. Some who had that believable presence that they could be the OT Vader embodied by David Prowse and JEJ’s voice. Someone with the physique, acting chops, and has that dark brooding / dual menace about him. For me Hayden never had that, I never believed the guy could be OT Vader (that’s not necessarily Hayden’s fault; as that is more on the casting, writing and direction).
Good call. Highlander fan?