Actually, Twenty Years After comes before The Count of Monte Cristo. I read the whole works as a young teenager, probably about the right time. The Three Musketeers was great, and Twenty Years After was pretty good too, but the third, Ten years Later, (which is a supernovel comprising three novels--The Vicomte de Bragelonne, Louise de la Valliere, and The Man in the Iron Mask) was full of politics that were pretty opaque to me.
I reread The Three Musketeers in 1992. It's still good reading--romantic, flamboyant, with a lot of social digs, some humorous, I missed the first time. Maybe I should see how they look now.