I plan on buying the DLC sometime but out of curiosity which ones do you think add the most content?

The one that adds the most is easily repentance but you need the other DLCs to make it work anyway.

Yeah, I know, I was just curious. That seems to be the opinion i've heard a lot, is that repentance is the best. What would you say the worst is? I know afterbirth+ has a really strange price of 11$

At this point, all of them are great because they add good content and complement each other. However, I don't think there was ever an Isaac DLC that was not controversial during the release. Afterbirth and Afterbirth+ both had a lot of bugs and stability issues, not to mention Afterbirth+ introduced the infamous Delirium. Repentance nerfed many items and was much harder to the point that it felt like a completely different game. I remember many people just straight out refusing to buy repentance because of that. So the worst Isaac DLC? Every DLC at the release. (aside from repentance+ which I can't speak of since I don't own the game on Steam)