So many to choose from but the ones that i have truely despised (rather than the ones i just find/found annoying as i'd be here all day!):
Stacey Slater (Eastenders) - i didn't like her from the off (if your going to introduce a long lost relative make it simple, not your dead nephews daughter who no one remembers!). Between her screeching like a banshee every 5 minutes, slapping everyone in sight (yet never seemed to get a good slap back 9 times out of ten) and still being the victim in any situation going (i killed him but i'm the victim!!!) she just always seems to get her own way which always screams of favouritism by the producers which is annoying in itself. Also she was milked for all she was worth in her first stint and for us viewers who couldn't stand her there was no escaping 'poor little Stacey'. She might not have featured quite so heavily since her return but by the sounds of it next year will be another 'isn't Stacey fantastic, lets centre everything around her and her mood swings because EVERYONE loves Stacey Slater'. They should have thrown away the key when they had the chance!
Kirsty and Kane Phillips (Home and Away) - absolutely vile the pair of them. Kirsty had always been my least favourite Sutherland, she has to have been the most selfish character to ever appear on TV yet she was another 'i'm always the victim' in any given situation. When she married the man who raped her sister for some reason she became some sort of heroine and Kane became Prince Charming and Dani (the actual victim) was painted as the baddie for daring to stand in the way of Princess Kirsty's happiness (how selfish of her!). As for Kane he was a rapist but forget that little detail as he was suddenly turned into Saint Kane, all round pillar of the community and superhero. Glad later producers ruined their happily ever after by bringing Kirsty back without Kane. They didn't deserve happily ever after.
Gypsy Nash (Home and Away) - no one ever seems to have a bad word to say about this character but i just hated her. She was so smug 24/7 and always got her own way despite being quite a nasty piece of work. Her return proved she was still as atrocious as she ever was.
Ruth Osborne (Hollyoaks) - sucking on lemons gold medal winner! Why they killed off the brilliant Natasha (ending the will they/won't they with Kurt) and replacing her with her inferior, dull cousin was a huge mistake. Plus she stayed for ages and when you thought she'd finally left she'd keep popping back just to be an even bigger cow than she was before!
Jo Sugden (Emmerdale) - she was a right cow to Katie (almost crippled her once) yet just got away with it scott free. She kept conveniently 'losing' her clothes to distract viewers from the fact she was a terrible character played by a terrible actress.
Chas Dingle (Emmerdale) - i hate overused characters and Chas has been overused to the extreme. Shes another nasty piece of work but because she does it with a few funny one liners shes considered salt of the earth.
Brendan Brady (The Brendan Show) - Hollyoaks is going through a very dodgy patch at the minute but at least all the characters are doing something. During Brendan's reign absolutely everything revolved around him. His mannerisms were annoying and the whispering got right on my last nerve. His cheesey declarations/monologues about how much he loved Stephen every five minutes made his 'tough guy' status very hard to take seriously (even less so when coupled with his bizarre helicopter dancing in Later, cringeworthy).
Izzy Hoyland (Neighbours) - i loathed her. Not only for breaking up Karl and Susan (for which i've struggled to like Karl half as much since) but the fact she lied and lied and lied a bit more and seemed to be ever the more popular for it. This was around the time for me Neighbours went downhill fast and Izzy seemed to be flavour of the month (or four years or however long she was there). Not to mention that her and Karl's odious affair was coupled with a secret love child concieved in the stupidest possible way. Holly may have turned out quite sweet in the end but i still don't like to think of her as part of the brilliant Kennedy family.
Sam Mitchell - irritating personified. She was an ungrateful, sour, bitchy nuisance and so inferior to her brothers. She was less annoying played by Kim Medcalf but the essence of the character was still there so even she was a bit annoying. She was just an all round weak character.
Special mentions to: Frankie Osborne (Hollyoaks), Steph Dean (Hollyoaks), Steph Scully (Neighbours), Sam Fitzgerald (Neighbours), Debbie Martin (Neighbours), Josh Barrett (Home and Away), Lisa Dingle (Emmerdale), Evie Maguire (Home and Away), Peggy Mitchell (Eastenders) and Chloe Richards (Home and Away).