1. M - Always loved that letter. Maybe because both times it was the first letter of my name.
2. S - Simply because the most words start with this letter and if they don't then they are likely to have it (perhaps the most common consonant) and because it looks like a snake.
3. R - Just like the sound of it :P
4. X - Very mysterious
It depends on my hyperness. If I am hyper then it's a relatively high pitched and fast 'R'. If in normal mode then a neutral, long 'R'. If tired or not happy then a quick but very low-pitched 'R'. Lol!