Guarantee is some thing not possible in the internet.

Say I guarantee your site first position on Google, for the keyword "XYZ" and you get the first position in like 1 week.

What if the guy who did your SEO gets another client, asking for the same service for the same keyword. Will he tell him that "I did another guy's work for that keyword"

No he will not. He will keep on the work and mess the both of you up.

Best service is the DO IT YOURSELF service, accompanied with USE YOUR COMMON SENSE