This question can have several answers, depending on what type of hate we're talking about. If it's
annoying, then the Zubat line due to being just so damn common. They poison you and will most likely confuse you at the same time, and especially early on, they're probably faster than your Pokemon as well. The Tentacool duo shares this exact same problem, poison and all, plus they can Wrap you, thus taking out a really horrifying chunk of HP each turn, especially if your Pokemon hurts itself due to confusion. At least the Tentacool line isn't as fast, and have several more weaknesses. Geodude's line will almost certainly explode on you every time if you don't kill them in one or two hits, probably taking out one of your Pokemon with it, so caves are just generally pretty frickin' annoying places. Ludicolo has a couple of those damned flinch attacks and a confusion-inducing move, plus really only two workable weaknesses.
If it's
sheer strength we're talking about, then several Pokemon fit this bill as well. One of them would have to be Bronzong. Only two weaknesses, one of those almost always being negated by its ability, freaking high defense stats, and a great ol' fat load of Special Attack due to being a Psychic-type. Spiritomb and Sableye have no weaknesses and several immunities due to being Dark/Ghost type, and each of them have half-decent attack stats to boot, making them not just good for walling opponents. Made even worse if it's a hacked one in a Wi-Fi battle that has Wonder Guard as an ability, thus only making weather effects, Spikes and Toxic Spikes, and Stealth Rock even remotely affecting them.
If it’s trying to capture, or at times even just to
find a Pokemon, then pretty much every single legendary out there fits the bill. For finding them, just about any shiny Pokemon fits the bill. Thus, getting a legit shiny legendary or starter is a virtually unforeseeable task. The only exceptions are the event shinies and the Red Gyarados. Spiritomb’s also a doozy if you’re not very popular.
Lastly, it’s just a matter of appearance. Immediately, Jynx and Mr. Mime come to mind. Enough said, really.