I find the dream, bell saving, and the weeding badges really hard. I can't be bothered looking at hundreds of dream adresses, the weeding badge is near impossible unless you neglect your town, and the bell saving one just takes time and lots of selling ;___;
The streetpass one feels like it was designed for the original Japanese player. The friends that I have that visited Japan got their badge the first day there. On the other hand, I get no AC streetpasses unless I go to a Nintendo hotzone and even that is rare nowadays.
I think for me, the hardest badges for me to obtain would be the Turnip one and the gold bellionire one. I don't think I have the patience enough for either of them.
Its actually pretty ok to get if you have a friend with a high turnip price, and Joan selling at the same time on the other. With this, it only took me and a friend about 2 - 3 days to get the gold badge. Was it hard work? Yes... but you can get it with some effort in your own time, piece by piece, if you have another copy of the game and another 3DS.
I'll probably never get the Street Pass badges either. It doesn't bother me so much since I know the badges are optional and that might be a feature intended for people who Street Pass a lot of people or kids who have friends at school who also play the game.