Eduardo I think it really depends which sport or event you are betting on, for instance in my opinion
Bet365 is by far the best bookmaker to use if you love horseracing. They give best odds guarenteed, whereby if you take a price early in the day and it goes off at a longer price, they will give you the longer price.
If you like fun bets on trivial things,
Paddy Power may be the best selection. They have numerous random markets on reality TV etc
For Football I think there is a muchness of a muchness, ie everyone is similar (but its best to search around for the best prices)
My suggestion is that you need to decide how much you want to bet, if you want to start betting substantially (IE every day) it might be best to open account with numerous bookmakers AND by doing so you get some good offers, including free bets.
If you want to bet at the weekends then choose a bookmaker that you like the website of, go on each and see which one is most accessible and has the features you wish to bet on. Once you have done that come back on here and follow our link to the site so we can take a little bit of money from the bookmaker in way of sponsorship.
Please feel free to join the debate in whichever area of the site you wish and most of all have fun!!!