In Maths, edexcel gcse is harder than igcse. CIE igcse I do not know but I’ve heard really bad things about it.
You have to know most formulae by heart for gcse whereas on igcse you get them given to you, but the questions tend to be slightly harder on average. Most people find memorising the formulae quite difficult so that’s what makes gcse harder for them. If your memory is good, igcse is even. Though gcse has a wider topic range though, and is particularly deeper in statistics, though not as deep in algebra. It also has no calculus at all. Overall it’s harder. And that’s coming from someone who did edexcel igcse 9-1 in 2018.
English, gcse is harder. Igcse is designed for fluent non-native speakers. Gcse is designed for native speakers. Enough said.
Sciences, I think they are pretty much even.
Don’t know anything about the others.