I enjoyed MW 3, but it had the least staying power for me than any post-CoD 4 title in the series. This is because it is essentially a revised MW 2, and while all CoD games are very similar, MW 2 and 3 would be hard to distinguish for someone who isn't as familiar with them. I think it served a purpose, as MW 2 is a horribly broken game, (admittedly, that's also part of the reason it was so fun) but the "been there, done that" feeling was hard to shake off.
Black Ops 2 is a pretty good game, and was definitely fresh when it first released; Treyarch is pretty good at shaking things up. I don't revere it as much as the original Black Ops, but it is a solid title and I definitely prefer it to MW 3. It's not without problems, but to me, it has compensated for its shortcomings.