Bus is probably the cheapest but the slowest and (IMO) least appealing.
Train travel can be reasonable, especially if you're under 26, although taking the Eurostar from London to Paris is always pretty pricey. Trains are probably the most convenient.
Air travel is often cheap (check whichbudget.com, ryanair, or easyjet) but there's the pain of getting from the airport to the city center, which is rarely fun and along with security, delays, etc. ends up eating a ton of time.
there's no one way that's best... just choices to try and find which fit you.