HELL NO.
The hardest GCSE is Edexcel GCE O levels.
Just compare the subjects, for example Edexcel's GCE O level Pure Maths is more difficult than AS level Pure Maths and definitely heaps harder than CIE IGCSE Additional Maths.
The others available (not sure about difficulty) are CIE O levels, CIE IGCSE, Edexcel IGCSE, Edexcel GCSE.
Of course there are the other boards AQA and OCR.
What you should know is CIE A levels are the hardest A level, a lot harder than Edexcel, AQA, etc. However, all of them, even OCR, have similar content. It's just that some CIE A level subjects go a lot further (in syllabi), and the exam structure isn't modular.
Sad that the universities may not recognise this.
People who do Edexcel obviously will disagree with my comments, but anyone who has tried both, or switched over to Edexcel A level from CIE A level, or done CIE A level (non-soft subjects) can verify this. Even educational counselors and teachers should know.