Picking the best really is personal preference. Arnold no doubt has a legacy that is going to be hard to match but claiming he had the best physique really has a lot to do with personal preference. So much can go into, size, proportion, vascularity, striations, etc. Just take a look at the picture below and look at all the different types of physiques.
A physique I always applauded is Stan McQuay's. He's not overly freaky but has nice symmetry and cuts.
Another person not to overlook is Ronny Rockel. Again, these are all my personal preferences. There's just too many to name.
If you're putting physique aside and just going for the most dominant, it's a toss up between a few of the already named (Ronnie, Arnold, etc.).