The best mystery I've read is The Speckled Band. The author do well enough for me to claim it as the best is that it's unexpected and suspenseful. I tried to do the same with my mystery story but I couldn't find a clue that seems random. I also should've included a murder.
Here's a great mystery starter:
My fellow Watson,
I can't tell you my name nor where I live. I'm being hunted down by my enemies so I must run. Whatever you do, do not get involved. They'll hunt you too if you do.
Can't say much.