AC1: Good not great. Repetitive but not boring. Getting mildly dated. Can skip for budget reasons.
AC2: Introduces new protagonist (Ezio) and new setting (Renaissance Italy). Beautiful setting and much improved gameplay. Get the gold.
AC: Brotherhood: Several tweaks and improvements on 2 and is the epitome of what a gaming sequel should be. Probably the best entry in the Ezio trilogy. And arguably the best true Assassin's Creed game. Get the GOTY version that has the extra content. OMGSOFTCATS SAYS - someone below said to not get GOTY! so confused
AC: Revelations: Step down from Brotherhood but still good. Adds unnecessary crafting elements and the cultural differences of the Constantinople setting makes it feel less Assassin's Creed-y. Don't buy the deluxe version if you buy this one at all - all it does is give you extra multiplayer junk. OMGSOFTCATS SAYS - someone below said get GOTY!
AC3: Weakest entry in the series to date. Flat uninteresting setting with flat uninteresting characters. Not a bad game but fans and non fans of the franchise at best will walk away with a that was okay kind of vibe. If you do throw your money at this get the deluxe version because the Washington DLC has some of the best gameplay and the main character is a much cooler badass with wicked cool Native American super powers.
AC4: Arguably the best Assassin's Creed game to date. Tons of really cool pirate action and lots of positive gameplay improvements. Which means the only thing wrong with it is that it's an Assassin's Creed game that's not actually strictly about, you know, an assassin. DLC is more of the same pirate-y fun and a really good standalone game.