I prefer the tough ones TBH - and long ones give a chance for more people across different time zones and work hours a chance to get involved - which I think improves the experience
A series of quickly solved challenges is hard to keep up with as well - I can't always have PPrune always open for example
Plus I learn so much from the thought processes and immense knowledge of posters when they explain their reasoning for a guess/answer rather than just give a name/location