I like my Spurcycle and Incredibell Adjustabell 2 bells. Both only ring once per strike, so I generally ring them twice (not a postman) when passing. Easy to do because I mount my bell under the lever of my drop handlebars, so it's within reach even when I'm on the hoods. The Spurcycle sounds better (*chef's kiss*) but the Incredibell is about as loud, a lot easier to mount and move, and works better when it rains. Oh and it's generally under $10.
For flat handlebars pretty much anything under $10 seems to work the same in my experience.