What is the best and fastest way to get from St Thomas to beef island?

Define what you mean by "best".

Fastest way is usually to fly. Private charter flight (most expensive option) will be quickest as the plane will leave "when you are ready". Commercial flights (usually cost less than private charters but less than ferry) will have the same amount of flight time but how long overall travel will take depends on how flight schedules mesh with your arrival time in STT. I'm pretty sure Cape Air, Air Sunshine, Seaborne, LIAT are just a few of the airlines that fly that route.

Other options are ferry or private water taxi service (cost is more than ferry but probably less than commercial flight). Travel time depends on which ferry you take. Go here for more info: http://www.bestofbvi.com/info/info_bviferry.htm (or do a google search for BVI' Ferry schedules - lots of links).

In season, Cape Air flies from STT to EIS a couple days a week. Last minute fares can be very reasonable. Our friends were planning to take the ferry, then discovered that flying would cost about $100/each way, which about equalled taxi, ferry, taxi, and saved many hours travel time.

The other airlines mentioned above fly the SJU route, but I'm not aware of them flying to STT.

If there are enough people in your party, chartering can also be a good option.

Air sunshine also will fly the route STT to EIS - cheers