Personally im think your asking the million dollar question. Its not just about becoming number one, but also maintaining that position. Whoever you speak to make sure they maintain good practices or your number one spot will become some number I cant fit on this page.
Lots of people will claim to make you number one but make sure they are using good practise it not that easy for a multitude of keywords or key phrases and any unethical stuff the engines will not tolerate it for long.
A good SEO company is Oyster-web, they partly work on a targets i.e. You pay a bonus if they get you a to an agreed position but they charge a bit
Good practice thats the important thing or the short term gain some people will get you will be just that short term.