Cheapest is hitch-hiking, but not necessarily the most reliable or safest. Easiest is helicopter, but hardly the cheapest.
And somewhere in between is going to be the answer. The cheapest won't be the most convenient. The easiest won't be the cheapest.
This stuff is why I keep a notebook for trips, to keep track of it all.
I look at one-way rentals.
Roundtrip rentals plus parking.
DCL transfers either way. (we prefer from the airport rather than WDW resort b/c resort gets you there so relatively late)
Limo/towncar/SUV service.
Some sort of airport/port/cruise/etc service.
Look at the costs of each for MY trip specifically. Decide what sounds best that time.
We've now done DCL transfers (from resort and airport), rental car, limo... All have good parts, all have bad parts.
The incredible convenience of not having to worry about the luggage offsets the minor expense difference (given size of party/etc)
See, I don't *worry* about luggage.
If I'm taking DCL bus from resort or Hyatt Regency MCO, the bags are taken from the room and I see them in my stateroom.
If I'm taking a limo, I can EASILY get Bell Services to get my bags, take them to the limo. Limo driver hefts them in then out at the port. Porter takes them. I see them in my stateroom. I'm actually LESS worried about them because I know where they are; they are in the trunk.

I don't have to lift them at all.
If I have a rental car, I can have Bell Services take them down to my car which I've brought around, and they put them in the car. When I get to the port, I have them take the bags out and I see them again in the stateroom. Or if I REALLY feel like it, I go to the rental agency, unload my own bags, get on the shuttle (handing bags over to the driver), and again, they go to the porter at the port.
To me, having the bags leave my sight 45 minutes before they otherwise would just isn't THAT big of a deal to me that makes me want to pay $35 pp each way to be on a coach bus. (dad was a Greyhound driver, I spent ages of my youth on the bus during custodial visits, I am OVER the bus)