Rosin core solder. Flux is unnecessary.
Better than crimping because it's stronger, and muuch better looking.
Doing it's pretty simple. Use an acetylene torch (or propane, pretty much anything that'll heat it up). Insert the ring terminal over the wire, and heat both the wire and the ring terminal fairly evenly to ensure proper solder penetration. The metal should be hot enough to melt the solder. Use the hot wire/ring terminal to melt the solder, not the flame itself. Fill the terminal until the solder starts to pool on top, and then try and get it to have a smooth top. To help with this I normally leave a real small gap between the end of the wire and the top of the terminal, so that no part of the wire is sticking out. Grind off any excess.
Remember to put on the rubber ring terminal cover before hand, so you can slide it back over the ring terminal easily.