I use a pair of 6 cell nicad car packs in series. About 1500 mAh is common, cheap and available at any LHS. Gives enough power to start up to 1.20 size engine and is light weight. Mine are attached to the starter, gives around 12 to 14 volts to the started, depending on load.

Lead acid also give good cranking power, but somewhat heavy and need to be upright.

Gel cells are about the worse, with high internal resistance. Ok for smaller engines, but they don't seem to last very long.