Fastest is technically helicopter! However, I'm assuming you haven't got megabucks, so I'd recommend the National Express coach. https://www.nationalexpress.com/en
If you are arriving at City or Horizon cruise terminal, you could walk it to the coach station in 15 mins, otherwise its a quick cab ride. Coaches to LHR will drop you at T5, T4 or the Central Bus station which serves T2&3. Usually the same coach goes to all 3 stops. Cost is around £15-£20 (but can be cheaper) and the trip takes around 2 1/2 hrs. Book as soon as you can, as lots of cruisers use this service (I've just done a dummy booking to check prices in October(!), so if you are travelling this year, go for it!).
Simon