1st is we're going with the one's you need to unlock Jean. We'll go with Cyclops first since he's got a good leadership and he's super helpful if you get his AoA uniform to turn him into a villain.
Next is Storm because she's a great blast for SL. You can't have too many blast for SL since Combat levels area easier to defeat.
Then, Rogue just so that you can complete the requirements for Jean.
Then we proceed with the Stryfe requirements. Domino first because she's super fun to play with and great leadership. Fantomex is super close since he can deal bonus damage to mutants.
You don't need to T2 Beast to unlock Jean. His leadership is super awesome though especially if you get his uni.