This is a summary of my AI ratings based on my observations on what characters are capable of defeating. Aside from the AI coding itself, the character itself also has an effect on the AI ratings (something with bad AI could be carried by its moveset and/or gimmick all the way to the higher tiers). Even with my years of observations, these are still subjective and NOT absolute. In MUGEN, anything can happen, even boss level characters somehow losing to small hitboxes or superarmor. This isn't meant to be serious and is just something I do to pass the time when I've run out of other things to do. And keep in mind this is still in the context of AI vs AI (player vs AI will be something different, especially in the case of gimmicky fighters like Bospider).

?? GIMMICK: Can't really be defined by any level of testing. Can beat the best of the best only to lose to the worst of the worst. Bokosuka Wars
00 PUNCHING BAG: Nigh impossible to score wins with. Too weak or self-destructive. Usually meant to be bad on purpose. Potato, NES Silver Surfer, Weakling, punching bags
01 POTATO: Typically otherwise capable fighters held back by no/bad AI or are just too weak to handle any real competition. Zodiac (from The Untouchable), Ultra Magnus, Veku, way too many AI-less characters to count
02 JOBBER: Featherweights and characters that could be higher if they didn't have the uncanny ability to throw fights. Maybe they're just one-trick ponies that can at least survive. Baseballman, Sun Wu Kong (with IX's AI patch, Level 5)
03 MEDIOCRE: Not terrible, but leaves something to be desired. Still watchable and capable of winning (and even pleasing crowds) on a regular basis. Manto (BAL's version), Kung Fu Man (IronCommando's AI), Aladdin
04 ABOVE AVERAGE: Much more solid and exciting. The kings of the lower tiers. Golrio, Evil Q, Rario, Alien Green (with IX's AI patch, Level 5)
05 DECENT: The bottom rung of the middle tiers. Speed (NS' version, AI Level 6), Cloud Strife (AI patched), Bloody, Makai
06 SOLID: Middle of the road, and I mean that in a good way. Not too powerful, but strong enough to consistently win. Sparq, Agrias Oaks (Ouchi's version), the majority of Mass' characters (Hawk, Sledge, etc), Mecha Gorilla
07 STRONG: The best of mid tier. May struggle to win against anything higher, but will decimate just about anything below. Hatsune Miku (Yu-Toharu's version), Carnage (DarkLuigi's AI), Pazuzu, Rock Volnutt (MAITAKE's version), Cammy Custom
08 DEADLY: The bottom rung of high tier, but can still pull off some amazing victories and are powerful enough to at least hang on. Predator Warrior (AI patched, AI Level 2), Magic Dunker (AI patched), Pheus Mor
09 ELITE: The mid tier of high tier (if that makes any sense to you). Jyashinsai, Wooden Doll (7-9P), Ohga, Temjin Arrange (Ohga), Aleti
10 BOSS: The strongest a non-god tier can get. Azteca (BK's AI), Fernandeath (Yanagi's AI), Hulk (MGMURROW's version, Boss.cmd), Magma Dragoon (Snowwolf)
11 CHEAP: This is where any fairness officially goes to die. Broly (BoyBoyz's version, Boss mode), Code Name O, B-B-H-Hayato
12 OVERPOWERED: Even more powerful and firmly in the realm of the cheap. Monkey D. Christ, Himeido (AI Level 3, Block AI 5/10)
13 DESTRUCTIVE: Cheapness taken to the next level. Shelby Cobra, Rekka, Gaap, Jado
14 DEMONIC: Some of the most powerful and dangerous fighters even beyond the lower few levels. Master Big, Superman Hero, CCHouou, A-Kain
15 RARE: Possibly the strongest a character can get and still be considered beatable. Rare Akuma, Dark Donald
16 GOD: Borderline unkillable, but there's at least SOMETHING out there that can defeat it. Flea, Gatanothor, Honki Ultraman, Ahojingod
99 BEYOND: Might not even be killable at all. May kill you before you even breathe. Aka Oni, Exodia