I'm currently using K800i and its image quality is the best among the 3 phones mentioned. One downside of K800i is its rather disappointing video quality.
With a 3.2 megapixel camera, one would expect the video quality to be fantastic as well. However, the good people at SE decided that a resolution of 164 x 144 pixels will suffice.

Sound quality-wise, I personally feel that the N73 tops the K800i. When playing music with the speakers, N73 seems to sound clearer. However, if earphones are used with the K800i, the quality is pretty ok.
My colleague used to own a N80 and he faced a lot of problems with his phone, even after sending it for repairs and upgrades etc. The phone either hanged or couldn't make calls/sms. Not too sure if this applies to other Nseries phones.
Ultimately, it depends on what you are looking for in a phone.