As I've mentioned, that question is a lot like asking someone which is the best dog?
The answer depends on YOUR preference...not someone else. You have to visit the various sites or perhaps one like
www.nextup.com and sample all the different ones.
They have men, women, children, younger and older voices. Usually you will want to opt for the 22kh voices instead of the 16kh or 8kh ones.
Give a listen to the same voice but on different sites as sometimes bandwidth, background noise, how and where it was sampled all have an effect on the quality of the final output.
But, as I said...the ultimate choice is yours!
Best of luck in your decision(s)!