Some of my Thoughts. May not be the same to everyone else, especially if your familiar with the Route. I've based this on Routes that I think at the First time of Learning in Four Sections.
Complicated
8, 51, 58, 60, 100, 127, 154, 157, 178, 228, 322, 380, 386, 410, 424, 450, 484, B14, C10, R3, P13, X26
Middle Kinda Easy
1, 12, 15, 18, 19, 20, 35, 53, 47, 55, 65, 75, 78, 85, 88, 97, 99, 119, 122, 124, 138, 171, 160, 181, 188, 198, 199, 202, 208, 225, 249, 250, 265, 281, 284, 354, 356, 358, 401, 422, 466, 469, 490, B11, B16, N47, N68, N89, N171, R2, R11
Easy ;D
3, 5, 21, 30, 36, 57, 59, 73, 82, 89, 109, 132, 159, 172, 176, 261, 405, 436, 453, H22, N3, N21
Piece of Cake ;D
9, 10, 25, 40, 44, 45, 48, 54, 56, 61, 63, 64, 68, 77, 86, 87, 126, 129, 133, 136, 137, 148, 168, 177, 185, 205, 209, 227, 229, 320, 321, 415, 432, 468, 485, H37, N63, N64, N136, N137, P4, R9
Not Gonna do too many be here forever ;D. Note: Many have been mentioned before in other peoples posts!
Out of All of them I rank.
The Most Complicated is Route X26
1. There are only 13 bus stops also the Route is 21 Miles long.
2. There are plenty of turns and the section between Teddington - Hatton Cross is quite difficult to remember.
Middle Kinda Easy is Route 75
1. The section between Catford and Sydenham Via Forest Hill can take a bit of time to learn. Then you have the section around Penge East Station, Once you know that the Routes pretty easy.
Easy is Going to have to be Route 436
1. Becuase It runs pretty much in a straight line.
Piece of Cake is Definately Route 129
1. It does not even leave Greenwich!!!!
2. You can easily do half of the Route in Your Car ;D. Driving Down Woolwich Road without even realising ;D