We had a great 4 days at the Continental Palacete. Its just north of Placa de Catalunya, but if you want to be on Las Ramblas, their sister hotel (the Continental), is at the top, just south of Placa de Catalunya. Its a bit quirky(!), very pink! but is clean and the staff are very helpful. There is a 24 hr buffet, which you could live on, but probably won't! They also have free drinks (tea coffee, wine, beer etc) and bottles of water, so you can just grab a fresh bottle before going out for the day. I'd go back there no problem.
http://www.hotelcontinental.com/web/