AC1 is a frightfully boring and tedious game with dull story and wooden characters. You're not missing anything if you don't play it. Unless you care about Desmond's storyline, which nobody does.
AC2 as it's follow up completely blows the original out of the water. Ezio is a fantastic character that you can actual care about and relate to. Story is top notch. Gameplay is great and there's lots to do. Lastly Renaissance Italy looks just gorgeous. Just a fantastic, well polished game. Most people consider it the best in the series.
AC: Brotherhood expands on AC2's perfection. It's not quite as good, but still offers a solid story with strong characters. Exploring Renaissance Rome is a great gaming experience.
AC: Revelations drops off a bit. It's still a good game but has a much weaker story and felt a bit too short. Ties up the Ezio storyline.
ACIII was a bit too ambitious, and as a result is all over the place in style and tone. It also takes a LONG time to get into. When it's good, it's really good, but a lot of the game is a tedious slog. Connor is a return to more wooden, less relatable characters. Though he does get through by being the classic fish out of water. He does suffer a bit from Forest Gump syndrome, in that he's at too many major historical events to be plausible. Kinda strips suspension of disbelief.
ACIV is a return to the form of AC2. It expands on the best parts of ACIII without pretentiousness of its predecessor. It's arguably one of the finest pirate themed game ever made. Great ship to ship combat mechanic. Edward Kenway is a lovable jerk. Missions are fun and varied, there's lots to do. Just a great game.
I haven't played Unity yet. Hopefully it's fixed by the time it ends up in the bargain bin. It's unfortunate its severe technical flaws overshadowed the game. It shouldn't have been released when it was. As Miyamoto once said, a delayed game is eventually good.