I can agree with TrollHunter about KH2 having a better combat system while KH1 has the better story but then again I feel KH2 has the weakest plot in the series though I haven't played KH:coded yet.
KH1 is the most Disney friendly in the series, making the Disney elements feel important (same with the FF cameos) the rest of the series moves onto its own story that KH1 will introduce and Disney and FF elements will feel more like filler in the later installments.