I ranked all 20 of black's responses to c4 with a shocking #7

 All c4 responses ranked:

English Opening, or c4, takes control of the light-colored squares without committing a center pawn, and plans to stop black from occupying d5. This ranking goes through all of black’s responses to it. He can occupy d5 right away, go for the light squares, go for the dark squares instead, challenge the pawn on c4, or take advantage of it getting in the way of white’s light-squared-bishop, or let white get the center and attack it later, maybe transposing into d4 responses. Almost half the responses are middle tier and equal.

Bottom Tier:

This is the smallest bottom tier yet:

#20 No name (Nh6)

3rd time I put this at last. Basically it’s generally a bad move and c4 doesn’t do anything to justify it. Master level popularity (1 game that white one) and the engine supports this being at 20.

#19 No name (f6)

Waste of move that will probably become f5 anyway, and until then blocks in your knight. You might as well just play e5 right away.

#18 No name (h5)

Stops the kingside fianchetto white tends to aim for with the English Opening. But at the cost of your structure and tempi. After d4 you can’t even move out the rook, and you can no longer prevent white's minor pieces (or queen) from occupying g5 without f6.

Middle Tier:

Moves that equalize, but either don’t really play for a huge advantage or they are just dubious.

#17 No name (a6)

The bishop can’t even get to b5 since it’s blocked in, making this move pointless. The knight can still possibly go there, but when it’s the only threat you really shouldn’t be concerned more than you are about the g5 square. Play h6 instead. Maybe it supports b5?

#16 No name (h6)

A waiting move that stops potential habitation of g5 with white’s minor pieces. Otherwise useless though.

#15 Jaenisch Gambit (b5)

It basically goes for the Wing Gambit in reverse, only that extra move needed to play e5 makes it not fully worth a pawn, and without playing e5 you definitely won’t achieve sufficient compensation for it. I think Wing Gambit is definitely better for white, but I also think its reverse, Jaenisch Gambit, is too. This is an example of black challenging white’s grip on the light squares.

 #14 Myers Variation (g5)

This goes for a reverse Grob, but once white plays the intended Nc3 you aren’t really threatening the rook anymore, you’re merely threatening to cripple white’s structure.This also prevents Nf3, but that would get in the way of a potential aid against d5 anyways via fianchetto. This is an example of black countering white’s light-square-control by attempting to control the dark squares.

#13 Anglo-Scandinavian Defense (d5)

Originally I was gonna put this move in the bottom tier, but the c6 or Nf6 follow ups aren’t totally awful, though giving up the pawn and the center still isn’t really worth those tempi you gain. There isn’t all that much you can do with them.

#12 No Name (a5)

You are taking advantage of white’s pawn blocking the Light-squared bishop in, and can now get the rook out. But your opponent will have 2 pawns (or 1 pawn and one piece) already out before you get your rook out, making it a bit harder to maneuver. Still playable though. You can also play Na6 next.

#11 Anglo-Lithuanian (Nc6)

After d4 the knight gets chased around, and it doesn’t have the same level of counter play as Alekhines, but more than against 1d4.

#10 Caro-Kann Defensive System (c6)

I wanted to put it at 15, but unfortunately, I can’t because it’s technically solid objectively. This leads to the English Version of Ruy Lopez or the Catalan, where black is just playing for a draw. 

#9 English Defense (b6)

Just like white, you also want to control the center without committing anything. Could easily turn into some 1d4 lines, or if white continues his control without occupying strategy it becomes an even more tense, patient game of nerves.

#8 Agincourt Defense (e6)

Too high of a chance of becoming the Catalan, or similar types of positions. Everything else is fine though, especially French Defense Steiner variation.

Top Tier

Now we are at the moves that play for a win, and worst-case-scenario equalize. The bottom move in this tier may surprise you that I put it this high.

#7 No name (Na6)

That’s right, moving the knight to the rim is one of the best. This move is very underrated! Since white played c4, your structure is safe. This is the first time I ranked moving a knight to the rim higher than moving the same knight to the center. You can keep the knight here till you determine whether you should move it to c7 and go after d5, support a c5 advance against d4, or move it to c5. You also need to remember that the main quirk is the shock value you get without having to play a bad move.

#6 No name (d6)

Best unnamed response. In the event white plays d4, you get the English Rat Defense, with its epic stats for black. Otherwise you may wind up with the KID or Pirc type of set up. White is a bit more flexible here, but black gets a good position no matter what.

#5 Anglo-Dutch Defense (f5)

Will probably turn into the normal Dutch. A bit weakening, but by far the most aggressive move for black that isn’t brazen.

#4 Great Snake Variation (g6)

Takes advantage of white having exposed the a1-h8 diagonal, without any danger at all. Very good move.

#3 Symmetrical Variation (c5)

You can keep copying white for a while without white being able to punish it, which you can't really do against e4 or d4. It’s really annoying, and is a good way to wait for white to blunder without risking blundering yourself. Or you can deviate if you see a way to steal the initiative. Your choice. Would be at 5 if not for the engine though.

#2 Anglo-Indian Defense (Nf6)

Could turn into common positions from Indian game, but there are also similar positions without white playing d4 that can be a little hard to play, and black needs to know a little more theory for this than against 1.d4 if he starts with Nf6. Still great though.

#1 King’s English Variation (e5)

The reversed Sicilian. Black doesn’t get the same initiative as white does against the Sicilian, but he still gets great play. Note that since white gets the chance to get his knight out right away, black can’t open the board, but he can instead occupy the center. And be the first to complete development. There’s also the idea of trading his bishop for the knight that white will most likely move to c3, since knights are better in closed positions, and it may double white’s pawns, unless white plays a3. Both sides are great here.