Estate Agents don't determine the value of property so signing up with the agent who gives you the highest valuation could be a big mistake.

When valuing a property the agent should use their extensive knowledge of the local property market, particularly sales of similar properties, to accurately value your home and recommend an asking price and marketing strategy for you.

Of course the agent is competing for your potential sale with any other agents which you may choose to get a free valuation from so there is always the temptation to give you a "higher" valuation in order to persuade you to sign with their agency. 
Property valuation is not an exact science and different agents may take different views on your property but recently we have seen valuations that have raised our eyebrows to say the least. Valuations of 20% or more above what would be considered genuine market value in some cases!!

Seller beware!!

Unless your agent is giving you comparable evidence to support their valuation then you could regret your decision to list with them and end up having to drop your price significantly or even pay two sets of marketing outlays to switch agents at a later date. and are great sources of information but always look for "sale agreed" properties to check that you are not being misled by an agent who simply needs another property on their books.