For lowest jitter (phase noise) you need to have an oscillator that is tuned with a very high Q resonant circuit. Best would be a crystal or surface acoustic resonator for fixed frequency, or at the very least an LC resonant tank circuit if you need to vary the frequency.
The amplifying device associated with the oscillator must also be of a low noise type.
Ring oscillators, and various types of multivibrators using C and R as timing elements are just horrible for jitter. These trigger on slowly rising or falling voltages, and any thermal noise will interfere with the timing interval, and create jitter.
A ring oscillator contains a whole bunch of these horrors, so its definitely not a preferred solution.
Best bet would probably be to buy a commercial VCO that will have a specification for phase noise and jitter if its just for a one off project.
If you plan to build it yourself, it would take a great deal of testing to equal a good off the shelf product.