For my part i have done all the above + Fellsman, Dartmoor Discovery, Connemara Ultra in addition to a few LDWA events That cover more than 26.2 miles, DD and Connemara are road races therefore damned hard on the limbs .
Two Bridges sadly No Longer exists as a race.
Whilst i echo most of the above comments i would not recommend one from another as i find they all have a character of there own.
As to the WHW being the hardest footrace in the uk i have to reserve judgement until i have done them all.
Just pick any Ultra and you will enjoy it because that is the nature of ultra runners.