your topic is inviting trouble.
i think the cameras that you mentioned are great cameras and can produce very nice images. But u should really go down to the shop and try each of them out. Please take into consideration, budget/feel/lens/support/what you plan to use your dslr for.
This type of question pops up like hotcakes ever too often.
Please do use the search function.
you know what i think is the best ? your hp camera lah.
jokes and sacarsm aside
d300 is good. used it before .. love it
d90 is good .. i use it.. and i have it.. and i love it.
40d is good... my friend has it.. i've used it .. and i love it too!
50d is also good.. i use it .. my friend has it .. and i love it too!
so what's not good ? whats not good is the person behind the viewfinder taking the shots and saying this is a bad shot and i need new body/lens to make it better .