I haven't listened to enough of the Big Finish audio Doctor Who dramas (Wikipedia
) to know, but I think fan consensus is that both Colin Baker and Bonnie Langford redeemed themselves in those.
In a documentary on one of my Doctor Who DVDs, script-editor-of-the-time Eric Saward says after producer Jon Nathan Turner stunt-cast Bonnie Langford he insisted on having her do a screen test, and he was pleased to find she could play against her child star persona well, provided the script was letting her. But then all the writers just pumped out Bonnie Langford child star stuff (and Saward couldn't find a way to script edit them, apparently).
My impression is that when Colin Baker arrived Doctor Who was already overloaded with internal politics, with a script editor and producer barely talking to each other, and it was surprising the production cogs kept turning as well as they did. It seems so unprofessional, compared to today's TV making where roomfuls of writers and producers sit around big tables endlessly polishing everything months in advance of filming.
Trial of a Time Lord (Colin Baker's last season) was that production team's attempt to finally get their crap together. It has some really interesting stuff, including Robert Holmes's last script and Peri's controversial departure, but it all falls apart at the end (because Holmes's death prompted an eruption of all the buried politics).
I can't remember much about early McCoy, but I think late McCoy is great. It was becoming a more mature 90s show just as it got cancelled.
I think the new show should bring Kate O'Mara back to be the Rani again.