The easiest of the trilogy is Mont Blanc in both in terms of technical difficulty and physically. The Matterhorn is the next hardest, as although slightly lower, is a very physical and aerobically demanding climb.
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Mont Blanc (4808m) is harder than Aconcagua (6960m) and much harder than Kilimanjaro (5895m) The effort required on summit day exceeds that of running a marathon.
All said the Matterhorn is not an exceptionally difficult alpine climb if attempting with a guide (route finding can be tricky if attempting on your own). It just requires excellent physical shape and a skill set that allows you to move fast and efficiently on exposed rock.
Ben Nevis is purportedly the hardest mountain (and highest peak) of the Three Peaks Challenge. That's why people tend to do it first if they're taking the 24 hour challenge.
The mountain most widely claimed to be the highest unclimbed mountain in the world in terms of elevation is Gangkhar Puensum (7,570 m, 24,840 ft). It is in Bhutan, on or near the border with China. In Bhutan, the climbing of mountains higher than 6,000 m (20,000 ft) has been prohibited since 1994.