Wingte:
The ultimate colonizer, in the demo at least, is a medium transport. Fit it with basic equipment (ship control engines), a colonizing module, and fill the rest with cargo and/or supply (depending on the range you want). With level III cargo tech, you can make a colonizer that creates a colony II (200 million) on landing! Great stuff.
And, for you guys who just discovered putting a cargo module in a standard colony ship, there's really two options. The cargo module gives you a better starting population but you can also put a supply module in a get extended range. Very useful if you are reaching really hard for planets and systems. Also risky since you are putting your colony beyond help of your fleet.
First facility you build had better be resupply. Early in the game, though, this might be the only way to get colonies out there where the scout ships and no others can reach. (Scouts, of course, are also full of supply modules and so can go further than a warship...)