In general, any of the varieties described as being "burpless" are not bitter, or least not too bad. Some others you might try are:
'Diva' is reliable, productive, and has a nice mild flavor
Cucumber (Cucumis sativus 'Diva')
'Rocky' has small fruits, but it produces a LOT of them and they have excellent flavor
Cucumber (Cucumis sativus 'Rocky')
My current favorite is 'Swing'
Cucumber (Cucumis sativus 'Swing')
