There are some good actors thast have been mentioned but who seemed to have a bad day at the office when they did Who - not because they were bad actors but possibly because the script was wrong for them (Cotton in The Mutants was written for a completely different, non-caribbean actor for instance, and the actor really struggled with trying to cope with the lines as a result), or because they were given bad direction as to how to play the character (Richard Briers should have been told to stop it at once and play it properly).

However, there is Jenny Tomasin.

She was bloody awful in EVERYTHING I have ever seen her in, from Upstairs, Downstairs to Emmerdale. The woman just could not act.

I love Mawdryn Undead, but Stephen Garlick as Ibbotson is just gut-wrenchingly awful.

Dougary Scott in Hide. Everybody seems to think he was great for some reason. I just found his acting boring and cringeworthy.

Peter Kay in Love & Monsters.

To be fair, that wasn't really Peter Kay's fault. More the poor dialogue and rubbish monster.

I actually don't rate him as an actor. I've seen him in enough stuff and I've never warmed to him/found him particularly funny. In L&M, he was just awful.

I'm genuinely shocked that no one has mentioned Billie Poutin' Piper yet. She was terrible.

Don't know her name but the woman at the start of Asylum of the Daleks who summons the Doctor to Skaro. Her delivery of the line "Doctor who?" always makes me cringe.

Jenny Tomasin always gets a lot of flack but I think she was perfect as Tasambekar. She was supposed to be odious and creepy.

Is no one going to mention John Barrowman? Mind you, he's gotten considerably better at his craft over the years.

It isn't the worst by a long shot but to this day there is still something that annoys me about Alex Kingston's Let's Kill Hitler performance.

The worst performance I've ever seen in Who from an adult, I'm being kind to the kids, is the guy who played that kid in the Tennant Sontaron story. He was awful. I thought Rupert Graves made a bad job of his guest appearance too tbh.

I watched The Robots of Death the other day, Tania Rogers as Zilda gave an enjoyably bad performance, for those into such things. As for cast regulars, putting it kindly, all Peter Davison's companions had limited acting skills, but I would say Sarah Sutton takes the prize

Good call. I'd forgotten about that. I remember the scenes of her trying to cry were quite something to behold

Freema Agyeman, two scenes in particular. When the Hath dies.
Also when she stares at the camera, after the planets are saved from Davros.

The wasp bloke (who was a copper in ideal) in the Agatha Christie episode, buzz buzz.

The whole of Fear Her.

I would say Graham Crowden in Horns of Nimon, but its a guilty pleasure. Same as Paul Darrow in Timelash.

Kylie Minogue was terrible, as was Peter Kay.
I agree about the acting in Hide, I expected more when I saw the cast.

Edit....... just thought of two more Ingrid Pitt and Beryl Reid from the "classic series".

I found myself thinking ... I wonder how she got the role

Got to be Adric for me, bless him. Should never have been cast but we know why plus he was awe fully mistreated by Tom baker at the time.

I wonder whether she's actually all that bad or just seems bad because they've managed a sweep of excellent actors elsewhere. Can you imagine how Poul's madness would have ended up if portrayed during the budget-free Colin Baker years?

It's the shouty bit when she gets rebuffed by Jobel..will never like it! Alexis Sayle used to annoy me in that story but I kind of appreciate his efforts now and find it quite endearing. Maybe one day Tomasin will click with me. (Might actually watch Revelations tonight to see if I'm more accepting !)

Oh, I think I'm going to watch Image of the Fendahl and with that in mind I'll throw in Edward Evans who played Ted Moss.

Jenny laird in planet of the spiders is awful.

James copeland in the krotons.

Freema agyeman was abysmal.

But, yes, Cotton is the worst. I agree.

It's something terrible, isn't it

Crowden, no way.

Nimon was played as a pantomime (oh no it wasn't ) and he was the perfect panto villain.

Classic:
Adric especially, but also Tegan and Nyssa. I watched a bit of Castrovalva recently, and really wished I hadn't.

New:
Christopher Eccleston (he's great in other things - I just thought he was cringey as the Doctor)
Son of Mine from Family of Blood (ditto)
and - shock! - Carey Mulligan

All of the above I felt were too 'big' in acting style for their roles/dialogue and all made me cringe at times, even though they are good actors usually. Actually, I love Blink, but I really hated Sally Sparrow's dialogue & delivery.

Plus!
Karen Gillan all the way up until The Girl Who Waited
Jackson Lake's "assistant" Rosita in The Next Doctor

other than Rosita I must say I find that an odd selection when compared to many in the shows long history.....

I wouldn't even agree with Rosita. I quite liked her.

Christopher Eccleston really?

I suppose it's your take on the question. Some really good actors are being named.

Richard Briers for example is probably one of the greatest actors who has appeared in Doctor Who but I don't know anyone who liked his role in Paradise Towers. I wouldn't say this was due to bad acting though.

Ken Dodd!

Martin Clunes was pretty bad too