Both are pretty easy, but imo, teemo vs. singed is the hardest counter in the game. I have been playing a decent amount of teemo lately, and it's just ridiculous how hard i can shut singed down. When junglers come to gank, it's usually a double kill for me because I have so much farm/kills from singed.
Darius, on the other hand has pull/ability swing on you. He at least can do some damage to you... but yeah he's fairly easy too, but you have to be careful because if you are not darius can kill you quick. Singed is literally 0 threat to teemo.
Singed can't even farm against Teemo. Most singeds I play against as teemo are able to farm 1 wave, and then have to go back, missing the entire next wave or two, depending on where they are at. If a singed tries farming 2 waves in a row, he is most likely going back to his nexus as a ghost.