The first two are correct grammatically. It is a matter of emphasis. The last sentence is not correct. It has to say either 'much more' or 'much less' . You can't say 'much peaceful'.

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.