IMO. . . . Top 5 no order.
1.) Superman. His f23, his mix-ups, his damage off of 1 bar, his trait, and his lasers allows him to play any game, any match up, and any play style you can think of. This guy can literally do it all.
2.) Doomsday. Contrary to popular belief, if you fight a good doomsday, you'll find that this guy is a beast. With Mb Air Venom, and air grab, he controls a relatively good portion of his air. He also has a dangerously large hitbox on his f23 combo, allowing for some pretty good footsies. His trait makes all combos punishable mid combo, and he can punish any zoner with mb supernova. When playing at high level, your best defense against this guy is to play his game, and attack when the player slips up. Worst matchups are GL, Sinestro, and Zod. Those at worst are 6-4s that can be shifted to 5-5s with positioning, trait management, and wake up mindgames. His earth shaker is an added bonus.
3.) Martian Manhunter. Has the same things Superman has, but with a teleport. . . I don't care what anyone says, unless you're Ms.Cleo, you are not predicting that teleport to the point where you are punshing it. Especially if the MMH players wise up and back dash after the overhead teleport, making it that much more safer on block. His trait makes rushing him down a pain, and he has a solid mixup game.
4.) Zod. Best zoner in the game. And by zoner, I don't mean full screen projectile spamming. This guy can controll all areas of the screen, conditioning, you to counter by going to another part of the screen. And that's where the fun begins. If you are mid-full screen, you are definitely going to have some issues. No matter who you are. Zod's trait just adds to this, while also allowing you to rush down the character, completely switching up his play style, mid battle.With a decent cool down, High level zod's are able to control the battle at all times.
5.) Aquaman. Aquaman's spacing and 1 bar 35-40% combos are scary in itself. But that's just offence. His trait allows him to play a very defensive game. Stopping Injustice's core method of damage combos. Yes, you can pop him up, but the opponent knowing that's what you're going for, severely hampers your advantage of surprise. Playing aquaman defensively will awaken his true potential, and make every single matchup at the worst, 5-5.
Just recently got into fighting games, and I'm an addict. This analysis was made from my experience alone, and can very well be debatable. Injustice is a very balanced game, and these characters can be swapped and tiered infinitely. Check out my thoughts, and let me get some feed back.