Of the original series I only played God of War 3 and Ascension.
Loved 3, thought it was great, fast paced and awesome gameplay and locations. Thought Ascension was, besides the graphics, pretty terrible. Never played the first two though (or the PSP games for that matter), but based on what I played I vote for God of War 3.
Besides that I also played 4, bought it because even though I thought Ascension was bad I remembered 3 being great. I couldn't get into 4 however. After an opening that was pretty cool I got bored fast. Won't be getting Ragnarok because of it.