First and foremost, see what the local fleet is sailing. Ask them the same question that you are asking here. See if someone will take you out on the boats that seem to fit your needs.

DON'T MAKE YOUR DECISION ON WHAT YOU READ HERE! TOO MANY OF THOSE THAT POST HAVE THEIR OWN AGENDA!

Relative to what? A Laser? Then anything mentioned in previous posts will feel fast. If you are looking for something that is fast relative to the entire beachcat population, then your choices are more limited and tend to be more expensive. If you want a relatively fast beachcat (top portion of a mixed fleet based on elapsed time), then the requirement for quick and easy set-up might discourage you from a spinnaker boat, as that just adds time to anyone's set-up and take-down. Then you might end up with a fast, single-handed boat without a spinnaker...an A Class. Remember, TOO MANY OF THOSE THAT POST HAVE THEIR OWN AGENDA!

An interesting exercise is to check out USSailing's Portsmouth ratings. This will take you to the page that tells you how to use Portsmouth Numbers.
http://www.ussailing.org/portsmouth/howto.htm
The following link will take you to the most current tables.
http://www.ussailing.org/portsmouth/tables05/tables05mh.htm
It is used to allow boats of different classes to compete on handicap. If you were to use this handicap formula to find out the relative time to go around a race course where the corrected time is one hundred minutes, you would estimate the elapsed time by the DPN in minutes. A boat that has a DPN of 80 would take 80 minutes to sail the same course as a boat that has a DPN of 60 would sail in 60 minutes. It is an estimation of relative speed around a racecourse. Here are the DPN of some of your choices:

A Class 64.8
Blade F16 2-up 65.3
Formula 16 1-up 66.4
Formula 18 (e.g., Tiger) 62.6
H16 76.1
N 5.5 Sloop 67.2
N 5.5 Uni 69.7

Portsmouth is the most common system used for handicapping in the US and is based on years of data submitted by race committees from numerous clubs associated with the USSailing. There are other systems, but the differences are probably not significant for your decisions.

Seriously, talk to the local fleet guys. They won't be near as likely to steer you wrong, and will let you experience their boats before you buy. You might find just the boat you want that is now owned by someone that is ready for a new boat. You get to try out the boat before you put down your hard-earned cash and he gets a buyer for his pride and joy so he can afford to get a new boat. Win-win. The local fleet will want you to be happy with your choice, where those on this forum (like me ) may have other motives. Just my opinion.