Both had their strengths and i like them both equally. Halo 1 - The feeling of desperation against a relentless enemy whilst waging guerrilla warfare. The confusion brought up when Keyes disappears, the surprise of the Flood and the race to beat 343 Guilty Spark in stopping Halo was all really well done. Halo 2 - "Humanised" the Covenant, we learn their motives, we see Earth and her fleet for the first time (300 SMACS, 150+ warships etc) and the fight to save Earth begins. We also see the cracks in the Covenant society which leads to civil war. The only two things I didn't like were the Floods early appearance (perhaps should have been left till the Gravemind revealed himself fully) and the rush leading up to the cliffhanger. Just my views on the matter. I'd have to say maybe I like Halo 1 a little more but i appreciate both :)