What is the best cruise line for a 53 year old single woman traveling with a female friend?

Celebrity or Royal Caribbean

Celebrity.....

Royal

I would not go on a cruise line if I were you.

I went on Carnival and had a very good time and didn't even get sea sick like I thought I would.

I would go on Norweigen. Check out

www.ncl.com

You'll have a ball on one of their cruises.

Depends on what type of Cruise experience, budget, destination you are after.

Carnival tends to be a bit lower budget, more families & singles, larger ships etc.
NCL, RCL different experience and a bit more exp. than Carnival
Celebrity tends to be higher budget, more attention/better staff to guest ratio, smaller ships, etc.

There are also "cruises" that are on Rivers in europe which is a totally different experience.

Suggest you visit a few cruise sites and a few travel agent sites (e.g. www.americanexpress.com/travel) to get better feel for ameneties and cruise characteristics. http://www.cruisereviews.com seems to have links to all the majors which may help you narrow things down (or may make things worse given how may are there... 8-)

I think you'd be happiest on one of those Lesbian cruises.