I don't think Tom has ever given a bad performance in his whole career much less so in "Doctor Who". He was always excellent and NEVER boring. A few of his stories may have been above average. But even those stories have alot to offer in terms of entertaining the viewer. Some folks are somewhat surprised by the fact that I enjoy those average Tom stories moreso than the best stories from "NuWho"! They just have quite a few more appealing aspects about them.. to me personally. You just feel like, despite having a weak script...they just were more creative and worked harder to make the best of it back then. Technology has made everything so easy and a lack of work ethic is too prevalent these days. Perhaps not enough time is spent on crafting these stories now. Not enough attention paid to the smallest detail.
Perhaps the term "least favorite" should have been used instead of "worst performance?" The Ark in Space is my "least favorite" as I dont like the story monsters that could have given me nightmares as a kid. Nightmare of Eden was just less quality than other stories done that particular season. So thats my further two cents.
I don't think Tom ever turned Big Finish down, actually. It's a common misunderstanding of facts. I remember Nick Briggs talking about it. Apparently, Tom said something general about getting loads of bad scripts and someone thought he was talking about Big Finish, while Tom had no idea who Big Finish even was back then, so he was definitely not talking about it.
I didn't like it when he broke the fourth wall and said "Even the sonic screwdriver doesn't work", or whatever it was. It was too close to sending things up in a show that hinges on the suspension of disbelief. The director shouldn't have allowed it.
"A man's work is nothing but this slow trek to rediscover, through the detours of art, those two or three great and simple images in whose presence his heart first opened."