MW2 and MW3 are still my favourite CoD games. BO1 was shit for about 6 months until they fixed several very big issues, like getting an actual party to work, then it was a decent game.
BO2 is the worst CoD I've played along with what ever the fuck the first exo-suit game was called. Fucking bullshit hitmarkers from sniper shots to the chest, getting killed around corners as if every bullet is straight out of that Angelina Jolie movie and some of the most boring maps I think I've played on any CoD. WaW was good for 6 months then everyone and their mothers managed to hack it and treyarch couldn't fix those issues. In the end they stopped even trying once most of the players moved on to the newer games.
I didn't bother picking up Bo3 and the other one. If I wanted to fucking jump around like the people in a house of pain music video, I'd go play Halo.
But in the end, it all comes down to preference with this, I know a lot of people who love the treyarch games and hate the others and vice versa and it's just about which games they did well on and which they enjoyed the most rather then which game actually worked better.