Probably Four to Five.
Five to Six is usually on equal-footing but one thing I've noticed that I don't like about it is when Peri asks about the bats milk (wanted to put breast milk for a second then, that would have been embarrassing) and the Doctor gives her a cold glare and says 'Finished.....only enough for you'. Like he was annoyed about having to save her and wanted to make her feel bad. I think it would have worked better if he said it much more at peace, in a 'no big deal' kind of way. Let Peri be the one to say it's not right for him to save her at the expense of himself.
Though in one way it nicely prepares her for the 6th Doctor's more abrupt personality and slight chip on his shoulder about her.
But I think it always works better when the Doctor gallantly faces his regeneration, I find it much more satisfying. I never want to be forced to feel sorry for him, infact putting a brave face on it can come across just as vulnerable if it's done right. Like with Three into Four, the special effects are pretty non-existent, but that's overshadowed by Sarah's emotion and the Doctor's heroic attitude to the end.