OK we need to specify here are we talking about
Picard on the enterprise with his crew and the rest of starfleet vs Vader with his fleet (commander vs commander)
OR
Is this a physical fight were talking about?
Physical fight, Vader wins. He's a fucking cyborg powered by rage and hate, he could murder anyone he wanted to. Picard has a fucking plastic heart ffs.
But, as a commander, as a leader and tactician, there is zero question that Picard would hand Vader and the imperial fleet their ass in any number of ways.
Just think about how many TIE fighters one cargo shuttle could destroy just sitting there mowing them down, pew pew pew.
The Defiant against a swarm of TIE fighters would be absolutely epic. Decloak, burst lasers, recloak before their light even reaches the other ships, just absolute carnage.