If somehow the Darth Vader jumped back into time and fought the Darth Vader of the past, who would win?
Darth Vader before his arm was chopped off before the heavy armor. But at the height of his dark side power.
Darth Vader from the future with Armor
who would win and why?
Darth Vader before he was chopped by Obi-Wan was not at his height of Dark Side power. He's much more powerful in the dark side once he gets the suit and continues his apprenticeship under Sidious. Sidious even says in the Dark Lord: Rise of Darth Vader novel that Anakin was still "between worlds" (lightside and darkside), and that contributed to Obi-Wan defeating him.
The suit cut off his force potential overall, but Anakin was still a newbie with the darkside pre-suit.