I'd confidently put Ermac somewhere in the Top 5 space. The problem with whittling down a direct Top 5 is that there are many characters who have an argument for their placement. I did summarize Ermac's tools on a different thread so I'll bullet point them here for those who maybe don't know.
1) Anti Airs are among the best in the game. D1/D3/D4/St 4/B1/St 1/F3 can all AA for full combo.
2) Top Tier jump attacks. Ermac's Ji3 is a hair or two slower than Scorpion's but is just as Active. His Ji1 has l
long reach similar to Sonya's and his J2 auto corrects and is extremely difficult to AA with anything not named
Ball Roll
3) High Meterless Damage. In all 3 variations Ermac can net between 30-42% without using meter. In fact,
using meter in his combos is often not worth the extra 3-5% you may get.
4) He builds meter faster than a chunk of the roster. Doesn't matter what variation, Ermac's meter build is
extremely good. Since he doesn't need meter for combos he generally always has a bar for Armored
reversals or breakers.
5) He has a safe launching Mid that Anti Airs, confirms for full combo, can be special canceled and normal
canceled into an overhead that breaks most single hit armor in the game. F3 is literally a god send normal.
11 Frame startup, -4 on block, can be easily staggered with other normals or IASB for full combo.
6) IASB, while both versions aren't as plus as they used to be. They're still plus, still unreactable and one
still launches for big damage in the corner. Can also be used to combo mid screen.
7) 6 frame D1, 8 frame D3 that low profiles under some mids including D1's.
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Taking all of that into account, it's easy to slap a Top 5 label on Ermac, some may see Top 1. Though I almost wouldn't push it that far. The Top 5 contenders (based off of community posts/tweets/rants) Still filled with the likes of Cassie, Jason, Smoke, Sektor, Shinnok, Ermac, Jacqui (Some) Pyro Tanya, Sub Zero (Debatable I know) Kitana & Mileena. So at that point you have to break down each of the characters and how many winning/neutral/losing match ups they have.
Could you say that Ermac as a whole has more or less winning match ups than Tri Borg, Mileena, Kitana, Shinnok or Cassie? In my experience with Ermac I've found that he can contend against most of the characters in the game with at least 1 variation. Though the following have always felt like a struggle regardless:
1) Displacer Raiden
2) Mileena
3) Smoke
4) Scorpion (Hellfire)
5) Tremor (Crystalline beats MOS & Mystic. Runs even-ish with Spectral)
6) Shinnok (Others may debate me on this one)
Overall, I would say Ermac is certainly a top 5 contender, maybe even a Top 3. But with the way the game is in the current state, I'm not sure I could definitively state that he's #1. PERSONALLY I think Tri Borg fits that spot as the character with the most winning match ups. I'm not in the camp of, "Well each of his variations play differently so we should count them.---" no. He's 1 character with 4 separate variations. His are just more radically different than other characters. As a whole, I'd lean towards him over Ermac, though that is of course debatable.