In the CoD series I skipped WaW and MW3 (though haven't touched the single player for either Black Ops).
With AC I played 1 and 2, but when Brotherhood came out I didn't feel ready for more Ezio and Italy. I liked Altair and his locales more, but did like the improved gameplay elements of 2. Now that 3 is out, I feel like I have to play Brotherhood and Revelations first, but that's a lot of game to get through and still don't know if I really want to get back into Ezio's shoes. They really should have just used another ancestor instead of milking Ezio so much.
I was fine w/ more Ezio in ACB and really liked it (namely for more gameplay improvements) - but when I got to ACR, a few things went wrong for me. I didn't care for both the new Tower Defense mission types & Desmond's first-person puzzles. Also, I began to finally grow tired of Ezio. W/ all of this, I just shut off ACR. I don't think at that time that I was ready for "more of the same" from this series, after recently finishing ACB...and felt that I just wanted something more out of the series. I'm probably gonna have to eventually get back to ACR, so I can be ready for whenever I do get around to buying AC3.
Oh, yeah - Missed WaW, too. Forgot about that one.
I liked MW2's SP - but it wasn't as nowhere as great as COD4:MW.
During Steam weekend, I have tried both MP's for MW3 and Black Ops 1 - wasn't nada special there, IMHO. I did like one thing: MW3's Kill Confirmed mode in the MP, though. I can't speak on MW3's SP - just b/c I ain't felt up to playing the SP yet. I still ain't bought Black Ops 1, so I can check out the SP.