Hilary's answer is correct. You can't say "much peaceful".
However, you should also not say "the city one". I think the most natural would be to repeat the word life. So, "Rural life is much more peaceful than city life."
You could also say "Life in the countryside is much more peaceful it is in the city."
Finally, "peaceful" is OK but you could also consider "placid", "calm" and "tranquil" as options.