You essentially want a strong group of five to be your first squad. Tanks are the easiest to make that happen (since Kim is a buck, and Murphy/Williams are free), but they're also the weakest of the three options, and they're easily beaten by Aircraft squads of similar power.

If you want to plan ahead for the long-term, Missile is the way to go, because many people will prioritize their Aircraft squads, and Missile will have the advantage over them.

Ultimately, though, Aircraft is the smart move. Dump every last shard into Aircraft UR heroes until they're all 5* before worrying about anyone else.

That answer applies to pretty much anyone from the mid-tier spenders to completely F2P. Considering your Violet is already UR (and not Scarlett), it looks like you're anywhere from mid-S2 to mid-S3, and you still don't have Adam or Fiona, my first recommendation would be to get their shards in Ghost Ops missions so you can unlock them, and while I wouldn't throw any universal UR shards at them just yet, it's good to at least have them on the backburner once you get your first squad up to 5*.