I think that each pregnancy milestone you reach is the "best so far". Finding out you are pregnant is memorable, especially for those who were TTC. Feeling the little flutter of your baby adjusting and settling in for the nine month trip. Hearing their little rapid heartbeat for the first time... okay, every time. The first time you feel that little one move. The first time the daddy gets to feel his baby move, you'll melt at the sight of his face as he feels it. The ultrasounds that start to really take shape and you can see their little fingers and toes, see their cute button nose...
Then there's the birth - not so fun...
But let's face it - that is by far the best moment of pregnancy, when it's all over and you can look in the eyes of your little baby and feel yourself fall further in love with him/her than you were before, more than you ever thought you could love another human being... even if this one isn't your first. There is just something special about that first eye contact that will leave such an impression in your heart.
So - I'd have to say the best moment of a pregnancy is the moment it's over.