Yellowjacket is the best character there, but he can easily be farmed later. War Machine is the next best. However, I'd unlock Vision and focus on farming Vision, Ultron, and War Machine bios in elite stages.
The one good thing about getting Panther bios is there is only 1 elite stage where you can find them so 60 free is quite the boost. The bad news is, even if you unlock him it will take forever to get him to a useable rank and the rest of your squad will quickly surpass him.
So imo, get Vision then run Ultron (leader), War Machine, and Vision. That will get you through the game as they're 3 top tier characters. Eventually you'll also be able to farm Yellowjacket special missions. I would check to see how far you can clear. War Machine is actually awesome at that since Yellowjacket is a combat character in the mission (rather than a blast character when he's playable) so WM cleans house quickly.
Yeah, I've found YJ and War Machine to be absolute beasts. I only recently got Ultron and am now pursuing Vision/farming mastery mats. So basically, save the free bios for increasing the ranks of Vision/Ultron/WM and continue through the story. Thanks for the feedback!
Thinking of buying that ~2000+ crystals over 30 days pack so I can get costumes going on. I really like zombie Venom and Femme Captain America, but I'll probably wind up getting Ultron/WM's since they'll likely stay in the rotation.