Snips. He's ugly, he's annoying, his voice sounds like nails on a chalkboard, and he commits the unforgivable sin of being a comic relief character who just isn't funny. Besides, Buckball Season proved that Snails is a much better character without him.
Hard to say, like really hard. The DHX writers showing maximum effort when it comes to write away ponies I don't like. After season 5 the only two left that I really don't like are Snips and Spoiled Rich. Snips is just really badly written, but when it comes to character Spoiled Rich wins the worst Pony competition by a landslide.
I'm not a huge fan of Snips either, he is indeed very annoying. Spike isn't so bad though, I'm starting to like him a bit more.
I'm tempted to say Starlight Glimmer, but I feel that would be a little too easy. So instead my vote goes to Spoiled Rich, possibly the worst mother in Equestria. On top of being your stereotypical gold-digging trophy wife, she raised her poor daughter Diamond Tiara in the ways of being a complete and utterly heartless *squee!*.
Zephyr Breeze. That's an easy one. Worthless, pathetic and annoying. Flash Sentry is an obvious second choice.