The best letter to start teaching your child the alphabet is the letter that begins his name. Young children are interested in things that relates to them. But when you start teaching his initial, you start to teach in capital letters.
Most parents do this. Once they start teaching capitals letters, they continue to do so.
Capital letters are easy for children to remember and write because they are made up mainly of straight lines. But it is harder for children to switch over to writing in lower case when they've learnt the capitals. I've written on the pitfalls of teaching capital letters first here.