What is it about this place that means when someone asks for recommendations of X, loads of folk turn up and offer Y and Z instead? OP asked for sportives, not road races or audax, no matter how hard they might be.
So tough sportives... 'tough' will depend on your experience, but the daddy has to the be the Fred Whitton? 110 miles, about 13000 feet of ascent over 7 or 8 pretty monster passes. Some mega steep and some more gentle.
The Bealach Mor and Ullapool sportives are pretty good. First half of the Bealach is pretty each, but the Bealach itself is long and steep at the end, and the last 40 miles are full of lots of short and moderately steep hills that sap a lot of energy. Ullapool is similar, probably easier, but factor in the weather and it can be really hard.
Etape Du Dales always struck me as hard going too, never done it myself, but know a lot of folk who have and they all rated it as hard going.