Ok, just 4 (well, 5) asses I've noticed.
Claire in heartgold and soulsilver. I've actually had to reset the game on speedruns for this bitch. I'd start with totodile so I could get ice fang, and then I'd look up on bulbapedia all of the pokemon that can learn pretty strong ice attack around level 35-40. I'd go out and catch them all, get to Claire and still die!? I reset the game after 5 more attempts, and instead of a speedrun just said: "Right, I don't care if it takes me days, I am going to beat her!" So when I got to goldenrod, I spent days getting enough coins in the game corner for Dratini, and trained him vigorously throughout the game, along with my ice fanged croconaw (by that stage) and a couple of others, I then succeeded against Claire, and got to my second person...
Lance in heartgold and soulsilver: You have got to be kidding me, I just beat the bitchiest dragon trainer you can have, and here comes another dragon!? Only this time, the dragons are buffed, and there are only 3 of them!!! He didn't even have the courtesy to make them all dragons, he had a charizard, aerodactyl and Gyarados as well!? I didn't have one super-effective attack for his gyarados, and though I was prepared for Aerodactyl with ice attacks, he was a proper *********** and would OHKO - 2HKO my pokemon, every one, and then once I got the areodactyl down, there came Charizard!!! Not to mention, who the hell packs flail on a Gyarados!? What use is that, he doesn't even have an impressive speed stat, I'd get him down to the wire with my not very effective attacks and he'd flail on me, I brought out a mach punch and he just survived!? Uncool man, uncool. Besides, who the hell packs three of the same pokemon on one team, with different moves so your opponent doesn't have a clue in hell which is which!? Besides, the main thing I ended up using was spore (don't judge me) but then I found out that his Dragonite (one of them) knows SAFEGUARD!!! He must be the only champion or member of the elite four with such an annoying attack!!! I could go on for days about this stupid thing, but I'll get onto my third memory of the haunt (er)!
Lucian from diamond and pearl: I mean, come on! The 3 members of the elite four, were not efficient training, and I actually abandoned pokemon diamond because of him. (Platinum was easy as hell). I'd pick Diamond up again probably for a period every 2 or 3 months, try Lucian a couple more times, start the journey again (losing to my last people in the process) get to Lucian and BHAM, frustration and quitting. I remember one time very vigorously, because I was about to beat him and dastard Bronzong, but in my excitement I used an ULTRA BALL instead of a full restore!!! The game should really have some kind of box saying: You cannot steal a trainer's pokemon", especially in the elite four where it is probably an accident, rather than using up a turn for it. I am ashamed of that moment... That Bronzong is my worst nightmare, he would float there, mocking me because he was clearly superior to my party, another annoying thing was having to use some of your full restores because of the battles before, I ended up short on cash in the end, and then it ended. I lost. I had no money for full restores - max revives - nothing. I had to restart the game again, but I couldn't be bothered to run through it, at that point pokemon ended up seeming drab, and I wanted nothing to do with it anymore (until the Pichu with a ukulele swayed me some years later) and I put down the DS, switched it off without saving and pulled out the game cartridge. I put it in a draw somewhere (to be found there, waiting some years later) and left it. I still had fond memories of pokemon, and carried on with the trading card game, but that Lucian.. if I could have got a gun and shot him then I would, happily. However, the story doesn't end there, since I managed to beat him in late 2011/early 2012. Despite my affiliations with the aforementioned dragons, I enjoyed heartgold lots, and would time myself (though I'm still pretty damn bad at training, and the best I got was about 2 weeks XD) but I went through my various things one day and found the dreaded game, pokemon diamond. I went through the game again, with that Lucian and his weird metal robot seemingly fresh in my mind, and I got to the elite four, completed the first 3 and... Lost to Lucian AGAIN!!! My pokemon battling skills had improved massively since my days on diamond and I still lost!!! But, an amazing idea struck me, by then I'd upgraded to the DSi, but I never sold my original DS. This means that, since wireless works on both (but not the microphone, I used to have some kind of fun with chatter before it broke!) I could trade my buffed up heartgold team, over to Diamond (while keeping some all-stars in diamond) and beat that purple-haired maniac. I traded over Gligar, Graveller (who turned into Golem) amd my level 97 Scyther and the three legendary birds. Now, you hear level 97 Scyther and think, so, an overpowered team here... Nope. In-game, I decided to take some advice from Bugsy and went and caught a Scyther and trained only him, then I needed an HM slave, and grabbed a graveller, and then I caught and trained a Gligar later on because he's awesome, and there was a winning trio right there, which stormed Claire pretty well on that particular run-through, and Lance has a horrible weakness to stealth rock, let me put it that way. I tried to use Scyther as little as possible, mainly to train the others after I trade them back to heartgold, but of course I ended up Scyther! (Mine happened to have technician as well, which while new to me is seriously great in-game with him learning attacks like wing attack naturally, and then U-turn via TM) and then Cynthia was a breeze. Seriously, I still had quite a lot of trouble with Lucian, but Cynthia? She was way to easy! I then tried it with my diamond team + Scyther and boy oh boy. Scyther was there for Lucian (he could do almost do it on his own, 3-4 pokemon down and I could handle the rest just about) and then had my Diamond team, with five pokemon because I didn't use Scyther at all, challenge Cynthia and it was easy! They got their jobs the wrong way round, Lucian for champion all the way!
Anyway, this is my list of the trainers that should go to hell, pronto.
P.S I've read back through this thread, and hwo the hell am I the only one to mention Lucian!? He is the reincarnation of nightmares (ironically using psychic types), more so than Darkrai!!!