I honestly have lots of different tastes so it's very difficult to pick just one subject. My main subjects I guess are health, nature, animals, the paranormal, spirituality, folklore, history, science.
The ideas that I would leave in 'the trash can' are ones that have been written about numerous times. Searching on hub pages should tell you how many times a subject has already been covered - I think things like weight loss for example and SEO have been covered a lot. However, if you have a very unique angle on a subject that has been written about a lot then go for it. Or if you have the very latest information that no one else has then go for it as well.
But if you have loads of ideas then make a list and put them in order of preference, say from 1 to 20. Then work your way through the list starting with the idea that most appeals to you at the moment. Personally I tend to work on 2 to 3 hubs at the one time. If my thoughts ever dry up on one, then I'm usually fresh for one of the other hubs. This way I'm continually writing. Having said that, there are a lot of hubbers who don't like to work this way and perhaps you're one of them. But get those great ideas of your down on paper or on the computer and let your mind do the rest for you. Good luck.