The only time I went back was with Call of Duty 4. I hated World at War. Then again, I was extremely new to the series. I didn't start playing Call of Duty until a month before World at War came out. It felt like a step back to me. Modern Warfare 2 felt like a monumental step forward right up until people discovered overpowered setups. One-Man Army was a great idea, it was just abused.
When Black Ops came out, I loved the fact they took out Stopping Power. Looking back, they could have done a few other things right: leave out Second Chance, have a little more weapon diversity, and "earn" perks/attachments instead of unlocking them immediately. Even with Stopping Power missing some perk combinations were a little overpowered, Ghost Pro/Hacker Pro, Flak Jacket Pro/Tactical Mask Pro, Flak Jacket Pro/Second Chance Pro to name a few.
When Modern Warfare 3 came out, I didn't go back to Black Ops because I was addicted to Kill Confirmed and Specialist. I would usually rush to get a quick four or five kills, then slow things down and try to get the Specialist bonus. After I could finally get the Specialist bonus consistently, I felt unstoppable.
I haven't gone back to Modern Warfare 3 because I genuinely like this game. I stopped playing Modern Warfare 3 back in July, so I don't really see a point in going back to a game I haven't care to play in six months. At some point, I might pop it back in for nostalgia sake.
Black Ops 2 made me appreciate MW3. Which is astounding considering how bad the maps were. Nothing tops this collection of garbage maps which found a way to be worse. Other than the occasional Stealth Bomber or some loser using Dead man's hand, it's a boring, yet ok game.