play ac2, ac4, then go back and play ac1, revelations, brotherhood, liberation, and if you really want ac3. AC4 and AC2 are the best, AC3 is the worst, brotherhood and revelations adds more to AC2 (same character). You can jump into any of the games though and be fine, pretty simple background story (templars vs assassins).
wait for a sale, i recently picked up AC1-AC3 (every game) for around $30 on ubisoft christmas sale
I would agree with this. Although I wouldn't play AC4 first if you have any intention of playing AC2. AC4's in game options for exploration, open world pirate activities, collectables, side missions, island hopping, mini games, treasure hunting, fort takeovers... shit you name it. If you play that and then go to AC2, you may be bored out of your mind in AC2.
I would have to say the best franchise would go to Mass Effect, even though the ending to 3 was horrible. The original story line, really good gameplay tweaks, etc. I pick that over the following for specific reasons:
Uncharted 1 and 3 were mediocre compared to 2, and while they are great games, I am more partial to Sci Fi.
Bioshock 1 and Infinite were fantastic. If two had been shit canned and the second was infinite, Bioshock would win. Bioshock 2 was horrible.
The last of us only has one game thus fat, otherwise I would be naming this right up there next to Mass Effect.
Some notables for originality/gameplay mechanic changes
1) Dead Space
2) Alan Wake (They should make more - with less copy paste outdoor areas)
3) Batman Arkham games
4) Far Cry
Some up and comers we should watch, aside from The Last of us:
1) Dishonored
2) Destiny