I'll say Windsor Castle. It's an easy train ride from London and makes a lovely day out. The castle is impressively fortress-y on the outside and pleasingly grand on the inside. The town of Windsor is touristy but not soul-crushingly so, and you can walk down the hill to Eton too.
I really liked Chambord, but there's not as much on the inside as there is at Windsor. It's a beautiful place though.
MSM looks and feels like a dream ... provided you're there after hours. It's too hectic otherwise. Also, as has been pointed out, if it's truly a castle you're after, MSM isn't what you want.
If you decide to go to Windsor Castle, do check their website to be sure the State Rooms/Apartments aren't closed on the day you plan to go.