Or really best seat on an airplane. Of course most people would chose business class if it was a feasible option, but it is not the least expensive, so where do I think is best? When I'm traveling with my 2 year old, whose been on 80+ flights, I'm not overly picky about where I sit, I'm usually happy to board and get settled. That being said there are certain areas that I find are more convenient and one often surprises people; I like the back of the airplane.
Seats at the back are close to the washroom, a helpful thing when you are traveling with a young baby and frequent "trips" are required! It's also an area where few people actually want to sit (in my experience people want to sit close to the doors so they can get off faster,) so if the plane is not full, there is a greater chance that you'll have an empty seat next to you, always a treat when travelling with a child under 2 who is sharing your seat!
There can be a bit more activity at the back with people walking to the washroom and you may have to wait a bit longer to receive your drink during the flight (although on some flights I've been given a drink first because the flight attendant wanted to take care of Connor!) but all in all I think the back is best!
Let me know on twitter @flywithbaby where you like to sit when you are traveling with your child(ren)