There's no such thing as the best bang for the buck.

I suggest that you consider the following factors: your age (if you are 18, you probably don't want to be on the QM2, and if you're 93 you probably don't want to be on Carnival), your interests, the cost of travel to the ports of embarkation and debarkation, whether you drink liquor and how much (Regent is all-inclusive), what itineraries you would enjoy, would you like a repositioning cruise (a cruise, for example, from Portugal to Ft. Lauderdale that includes trans-Atlantic travel for 5 or 6 days), the size of the ship, the age of the ship (newer and larger ships tend to be more stable).

Do some homework. Borrow books on travel from the public library. If not, mosey into a bookstore and sit and read. Get on the internet, including Cruise Critic. Obtain catalogs from the cruise lines, but buy from a discounter.

Consider the cabin location. Would you be able to enjoy a cruise in an inside cabin (the least expensive) on a 4 month World cruise? Or must you have the Owner's Suite (very expensive) on a one week cruise?

Select your cabin judiciously. Cabins in the middle of the vessel have the lest pitch and lower cabins have the least roll (but remember that the upper decks have services.)

Good luck.