Loremaster:
Basically, it's what a wizard should be in the first place. He gets a lot of interesting abilities, and he is really a master of Lore. Maybe a bit overpowered since no caster levels are lost, and no significant abilities are lost, but it's not overpowered in the "can easily break a campaign/the party balance". You're Loremaster abilities don't steal anyones spotlight.
Radiant Servant of Pelor:
Okay, you become a undead turning and healing machine, and that's not for everyone. (especially since turning undead at high levels becomes increasingly difficult, despite all tweaks you might do). Overpowered maybe, but in the same sense of the Loremaster - Nobody will feel like you take the spotlight (despite your radiance!), but everybody will like you.
Shifter (3.0):
Turning into anything is fun. The base concept alone is great, regardless of the implication. That you can choose ridiculously overpowered monsters to turn into is just an added bonus (or is it tainting the concept).
Personally, I wouldn't mind redoing it with a better balanced variant based on the Druid Shapeshifter variant of the PHB II.
Thinking about it, I think these are also the only PrCs I have played yet. I think Planeshifter of the MotP was also very cool, but I never tried it out.