In terms of simple simplicity, a backflip is the easiest trick- what with the angle of the jump and everything, you're already a quarter of the way around already, and the momentum is pushing you that way. But it takes balls, and in general has more risks than other flips. Loops and front flips for me are easier mentally but harder physically. When I started flipping, I did front flips to get used to being inverted, then started backflips once I had the balls. You will usually go nuts with fear before you do your first back, but once you do it it's so easy it's funny.
'Cure blindness with a whore's spittle.' -Jim Morrison