You're probably right. Anyway its the big square at the top of the Ramblas with the fountains and where the busses stop

Please be very careful regarding personal possessions when you go to Barcelona, as a tourist you'll be the top target for pickpockets and bag-snatchers. Sorry to introduce a negative note, but Barcelona (and to a lesser extent Madrid) are infamous for this type of crime. Keep any sizeable amounts of money, jewellery, passports etc securely locked up in the hotel safe. Once you've sorted this out, and made sure that any money on your body is relatively inaccessible to thieves, you'll be able to relax and enjoy the city.

You mentioned you're going to be in an apartment for 2 weeks after staying in the hotel. Just in case you get bored of Barcelona (unlikely I know) you may wish to take trips to Stiges, Girona and/or Montserrat.
For the record my favourite part of Barcelona was Parc Guell - well worth the visit.

Thanks everyone for your replies