The best aspect of SR characters is that you can easily maximize their advancements in the game because they are in abundance when you summon characters on a promotional banner. However, not all the SR characters are insanely good because certain SSR characters, even without any advancements, outclass maxed-out SR characters without a fudge. A notable mention of an SR character that can compete on par with SSR characters is Ene and her weapon Pummeler. With better advancements in her weapon, Ene can dish out a lot of damage to enemies and can stun enemies with normal attacks. Here I assume that you can get her to 6-star and invest heavily in your gear and matrices.