Tigger
I've been to Versailles, although it was about 20 years ago. It was incredible!! The sheer size of the place was mind blowing. As we approached, the bit that I thought was the Palais was actually the stables!! Loads to see - as well as the Palais, grounds and various outbuildings that are open, there is also Le Grand Trianon, Le Petit Trianon and Le Petit Hameau (the little Hamlet where Marie Antoinette used to "play" at being a country shepherdess).
As for castles, much as I love most of the ones recommended, I LOVE Scottish castles. Fyvie, Stirling, Mey, Duart (on Mull - used in the Sean Connery/Catherine Zeta Jones film about an art thief), Eilean Donan (that was used in loads of films), Brodie, Cawdor and many others all seem to combine what a castle should look like from my child's memory point of view (turrets and towers), together with comfort and splendour inside.