Hmm... it sounds arrogant but I honestly can't think of anyone whom I couldn't have a stab at writing. Of course how well I do that is the reader's judgement to make. Sometimes I deliberately choose to write characters or pairings that I don't personally care so much for or ship, as a kind of challenge. And then usually in the process I gain appreciation to them, or some kind of insight. For example, I wrote Ianto/Jack for a charity auction fic and afterwards I felt like I've said everything I can about them and I'm extremely unlikely to tackle that pairing again. Or I wrote Hathaway/Hobson in a fic that was both het and needleplay and managed to convince myself about both the pairing and the kink. And then often these fics become the ones I'm most proud of precisely because they weren't easy.