I really like the Jandd mountain handle pack IV, Mountain Handle Pack IV

It has an excellent attachment system. I like the size a lot (around 6 liters). That's enough for the stuff you really need ready to hand but it won't allow you to overpack (a larger bag will have more of an impact on steering). With the strap, you take it as a shoulder bag so you always have your valuables with you when you walk away from the bike.

Edit: not waterproof but that's what plastic bags were designed to do; otherwise get an ortlieb.