I think Jensen Ackles
He did a great job in Under The Red Hood and he’s proven for years he excels at physical roles. I think he’d do a fine job, but I think we need someone a bit taller and maybe less familiar. I think most people would have a hard time getting past watching Dean Winchester talk to Batman.
I totally agree with antpadlo. It is hard enough for me to not hear Dean Winchester in Under the Red Hood (not that he didn’t do a fantastic job, he did!) I would love it to be a less known actor so we can associate him solely with Jason and I think it’d have to be someone younger as well.
What would be cool is to cast a 13-15 year old Jason and a 20-ish year old Jason who are either brothers or can pass as the same character 5-7 years apart. Then you can do an Arrow / Lost flashback structure with him as Robin and as Red Hood.