Worst solo character is easily Hsien Ko and bare Phoenix Wright with Frank Wack also in good contention for that spot. Shuma Gorath at least has combos off of throws and big damage... he can get lucky with a hit. He also has crazy limbs that can catch you by surprise and not a terrible ground dash.
I actually think Iron Fist is a worse solo character than Shuma too.
I heartily disagree.
Shuma-Gorath: No ranged ability without charging for 2 full seconds (lol in Marvel time), horrible speed, weak mobility, small ranged normals outside of c.M, no mix-up possibilities, nigh impossible to combo from hypers at mid-screen, has the potentially fantastic Chaos Dimension that is usually underwhelming in practice due to its very low damage (only 350K for the unblockable portion, which is what matters as an anchor)
Hsien-ko: Decent keepaway that would be good if not for random bombs, abysmal speed, fantastic mix-ups including popping out of her ground dash, a hyper that provides hyper armor, large normals, low damage
Phoenix Wright: Decent normals, some of the best utility hypers in the game, maya shield helps give him breathing room, can potentially destroy an entire team if he establishes himself (not easy solo)
Frank West: Not even bad solo. JWong has made a ton of comebacks with him. He has solid speed, great hypers, and good normals. Solid at getting in alone between pie throws and zombies
Iron Fist: Has a mini Bionic Arm, superb ground speed, big damage combos, great priority on his normals, armor options, and one of the best AA hypers in the game
I'm not saying that the non-Shuma-Gorath characters are good, or anything similar. I am just saying that they at least have stuff going for them. Shuma-Gorath is pure piss when he's solo. LLND has made a lot of comebacks with Hsien-ko. She's a terrible character, but she has some good options and abilities.