Depends what you like. I count Assassin's Creed as one of my favourite games but it would be true to say that the later ones just build on what was good from the first and so if you much of the enjoyment I still find in it is equally found in the following ones. I loved being able to move through these huge envirnoments so smoothly and satisfying which actually had loads of people populating them but that is all in the later ones. The setting is different though and although not as detailed as some of the later ones I really liked the whole Palestinian Crusader times architecture and feel.
There is more of Desmond and you can spend a bit more time effectively role playing as him and I quite liked that even if there isn't a tonne to it I felt like I really lived him more in the first one. The story and missions are all unique of course and might be worth seeing - I quite liked the feel of gradually finding the evidence which leads you to your target and this structure isn't used exactly like that in the later ones. The ending is harder and requires more skill to complete than the others and can get frustrating - I do think there's some really striking moments in parts of it though that I still can clearly think back to.