For Starscream, definitely Airhead as he hasn't been released to the PBC yet. Optimus and Ironhide can be found easier in the PBC, although if you're gunning for a 3* or higher I've noticed feature crystals tend to have slightly better odds. Soundwave was only released as a 3* or 4*, so I would say Airhead has slightly better odds on him, but your best bet for all but Starscream is grabbing some 3* crystal shards in the Last Knight challenge
Comments
I cleared the last Spotlight hard mode with a team of maxed 3* and one r2 4*, they're not as useless as you think