As a new player, you’ll need quite a few villains for SL. I’d recommend you farm all the Spideys rogues gallery villains first. Start with Sandman or Mysterio as they’re both very good for SL. They can both also easily get 100k in abx at T2 on combat villain and blast villain days respectively. Sandman can do it with some difficulty at T1 as well. Sandman at T2 can clear any stage he’s eligible for up to around 25 at T2. Mysterio is more suited to rumbles as his skills lock the enemies in place and target multiple enemies. He’s very good in blast rumbles and villain rumbles. After getting those two, Kraven is another good option. He has below average dps but very good survivability and crowd control. He can clear villain wave modes pretty easily. The shifter in those missions is vulture. He’s an amazing character who has great damage and survivability with 3 iframes. He can clear any SL stage he’s eligible for at T2 and can clear 100k in abx on speed villain day. In fact, he’s the best f2p option for that day in abx. Vulture and Mysterio are also extremely good in AC due to all the iframes they have. Rhino and Lizard aren’t great, but they’re worth farming for two reasons. The first is that you’ll get vulture bios from their missions. You can get an average of 7-13 bios per day. The second reason is that after getting them all to 6*, you’ll get a mega mythic uniform ticket from the quest pack. This is an extremely valuable item and as far as I know there’s no other way to get it without spending a ton of cash. After this, Crystal is another very good character to farm. She’s always an easy SL clear even at T1. Inferno is the shifter for the inhumans missions and he’s a great character with his uniform. I’d definitely recommend building him up and getting the uni.
Inferno is the best character from Special Missions but he's only a Shifter meaning he has a chance to show up during missions - not guaranteed. He is only in the Inhumans Special Missions. Vulture is the second best and he is also a shifter and only in the Spidey's Rogues Gallery Special Missions. After those two Crystal is excellent and probably the best easy to obtain Special Missions character. Mysterio would be after her then Sandman then Kraven. Those are the only ones really worth taking to T2 that can be helpful and even then all of those except for Inferno and Vulture are really only good for Shadow Lands and somewhat in Alliance Conquest. Songbird, Lizard, and Rhino aren't terrible characters, solid B Tier, but they're nothing to write home about. All the rest are pretty bad. Karnak can be decent with his uniform and White Tiger is decent as well.