All 4 are good unis choices. As for whom to built first, it depends on whether you are at a stage where you needs better cards or more ctp. Other than Thor and Hela, which you need to pick between one based on what you need, you can farm the rest at the same time.
If you intend to use uni tickets, use it on Hulk only since he is the only non-farmable character out of the 4. However I will suggest that you save that ticket for character that are semi-paywall or paywall.