Assassins Creed III will be the fifth game of the series...AC II, Brotherhood and Revelations round out the Ezio trilogy...they are WELL worth the time it takes to play them. It is easy to get caught up in side stuff though...especially in the later installments, where recruiting Assassins, sending them off on contracts, and even later...well, I don't want to ruin it for you, but lets just say there's a helluva lot to do in the later installments.
I'm more excited about there being an actual bow in this latest one than anything else...
Sir Edward, another cool aspect about these games is the interactions you have with key historical figures, for example- these are just a couple for each.
AC1 (1191AD): King Richard the Lionheart, Garnier de Naplus (Grandmasterof the Hospitalier Order), Robert de Sable (11th Grandmaster of the Knights Templar) and Sibrand (Grandmaster of the Teutonic Order).
AC2 (1476AD): Rodrigo Borgia, Leonardo da Vinci, Lorenzo de Medici
AC Brotherhood (1499AD): the Borgias, Caterina Sforza, Niccolo Macchiavelli
AC Revelations (1500AD-): Manuel Palialogos, Sakhulu, Suleiman the Magnificent
Each single player campaign will probably run you 10-15 hours depending on how deeply you delve into the side missions and other stuff...I generally end up putting in around 40 on each but man, it is well worth it.
Little stuff that comes in the later entries like rebuilding your stronghold gets upgraded to rebuilding the cities; the recruitment of Assassins not only affords you backup but can also be used to fill your coffers so you can outfit yourself with the latest upgrades in weapons and armor.
Mind you, the first one has none of the aforementioned minigames, but it is still my favorite one, simply because it is set during the Crusades, you get to hunt Templars that actually look like Templars, you bandy words with the Lionheart (and other luminaries of the time, like William of Montferrat) and you get to explore Jerusalem, Acre and Damascus. Three areas where a self-styled Crusader historian would dearly love to visit but know better than to actually go there.