Be it Doryuuzu, Erufuun, or Shandera, or whatever else might be annoying to you. Personally, I think Tyranitar is annoying to deal with, especially with the SandStorm Sp.Def Boost.
For this gen, it would be Roopushin, cause he got strong priority, good typing and awesome attack, plus its ugly. For all time, Blissey, and I guess I don't need to explain why.
Skymin, Jirachi, Togekiss, Dunsparce... all Pokemon that bring the game down to luck and not skill. Jumpluff is a nutter, but other SubSeeders are annoying also. Easier to deal with thanks to U-turn in Gen 4 though. PerishTrap Pokemon are very annoying - There's a reason it was frowned upon in Gen 2. Most Ghosts are a slight problem, but I'm a person who likes Fighting types, so that's natural. Wobbuffet and, to a lesser extent, Wynaut. Needs no explanation. CroCune is ProCune - It just doesn't die. CurseLax is also annoying. Finally, Dialga. He's far too good for my liking. If you want something doing, he can usually do it. What a typing.
Gen 4 - Breloom Instant sleep is just too much. While it can do so much after inducing sleep. Like Sub, SD, Leech Seed, etc. No Celebi is not a counter; Spore will inflict sleep.
4th gen : Haxxrachi, hands down. Haxxkiss is as annoying as Haxxrachi. Dunsparce lacks speed to threat me. :3 5th gen : Well...SubSeed Erufuun is a pain to face. I hate it. Shandera is annoying when it setups freely on Blissey ect. Scarfed ones are even more annoying because of Shadow Tag.
1st gen: eggy, could sleep, explode and had pretty good defenses 2nd gen: raikou, pretty much unkillable on the special side and was in general a pain in the ass 3rd gen: celebi, amazing defenses and could baton pass really well, probably should have been uber 4th gen: jirachi or shaymin, haxchi for obvious reasons and shaymin because it really tears through the metagame without latias or mence to stop it, amazing pokemon 5th gen: nattorei, man what a pain in the ass runner up: Gon2 with evo stone, dies only to stab attacks it seems, but is setup bait for some stuff so not as bad as nattorei who hits pretty fucking hard
Rotom maintains a top spot, though it's an entirely different Rotom and for an entirely different reason. O_O Hm... Nattorei's annoying but if I focus I can typically handle it. It doesn't help that I don't have a lot of Fire-users that I like enough to consider building a team around. Porygon 2 with Evo Stone probably takes the cake. I've always hated Porygon and its evos, so to see one used as a wall kind of annoys me.
Right now it is probably Garchomp in sand, that much bulk and veil hax just doesn't feel fair in the long run. Even then, no actual metagame pokémon can compare to the terror that is in-game Golbat.
Rankurusu @ toxic/flame orb Magic Guard CM Recover Psycho shock HP fighting unless you trap it early with shandera, its such a pain to take down. I can't find room for a phazer on my team which is a problem =(
In ubers, blissey. I tend to use a lot of special attackers and I just get flat-out walled. In dream world, there really are no specific pokemon that annoy me, though if I lost my u-turners, erufuun can be difficult.
Crocune is annoying due to it's sheer bulk.. Erufuuun is very annyong as is nattorie... In ubers probably latias due to it's ability to switch in on kyogre and set up...
My biggest annoyance is Breloom. I run either buly offence or TR, With bulky offense at least one pokemon is Spored, and he can pick apart the rest, though with TR, if TR is still up after the spore, Dnite OHKOs with Hurricane or Exeggutor just plain conters him with harvest lum.
Also that. http://pokemon-online.eu/forums/showthread.php?2354-Memorial-moments-in-PO-battles Posts #23-#26.
That Donphan is lolz ! Faced it a couple of times. Other than Sub-Twave Jirachi is one hell of an annoyer :x
Politoed, along with all the friends it brings: Kingdra, Ludicolo, Manaphy before it was banned. Rain teams are just annoying and unoriginal, and those pokemon would be mostly useless without Drizzle (barring manaphy, but I'm including Kingdra, I always found him laughable. I mean, can't even 2KO skarmory at +3 with waterfall? that's weak) Ditto, just cos it's an easy way out. Roobushin, I love psychic pokemon so he's mostly laughing stock for my teams. As a consequence, I think he's crappy and it annoys me when I see people use him. Urugamosu, I can't understand why people would want to use something that enters the battle with less than half it's health most of the time.
Roobushin, Kerudio, Machamp and Zuruzukin on Gen 5. I don't know why but Fighting-Type this generation are such dicks :/. Machamp is a classic because of his 100% Accuracy Dynamic Punch but the new ones are as annyong because Psychic- and Flying-Type Pokémon aren't that common on today's metagame (Fighting's main weaknesses). Our luck is that we have Rankurusu, Shinporaa and another Psychic-Types that are usable.