Prince of Persia (1989) - It was a brilliant game at its time. Definite classic, no question.
Prince of Persia 2 - It was graphically nice, but frustratingly hard. Overall, it was a decent game, but nowhere near the original.
Prince of Persia 3D - Haven't played it.
Prince of Persia: The Sands of Time - Aw, another classic! A true gem. The atmosphere, the story, the cinematics... everything is great. The only flaw of this game is the unhandy combat system.
Prince of Persia: Warrior Within - Truly underrated by most. Gameplay-wise it's superior to its predecessor. The story, the cinematics, the combat system are all good. It could have been a classic, but missed the chance with those one-liners, (some of the) sound effects, and a few other things. Despite the one-liners, I did like the voice acting though. Much better than HoMM 5!
Prince of Persia: The Two Towers - The only thing I really like about this game is the Dark Prince and his dialogues with the Prince. Oh yeah, and that cool daggertail. Other than that, this game is seriously rushed. It felt too short, and the story wasn't satisfactory compared with the previous two games. Although it uses the same engine, the graphics look worse for some weird reason.
Overall, it's just a SOT wannabe, in my humble opinion.
Best game in the series: The Sands of Time
My favourite game in the series: Warrior Within
"I saw the angel in the marble and carved until I set him free..." - Michelangelo