For SP I've played:
Modern Warfare
Modern Warfare 2
MW1 was good. MW2 was a bit over the top, and had a few annoying parts (the favelas...), but it was nice to revisit the characters again.
For MP I've played:
Modern Warfare
Modern Warfare 2
Black Ops
Black Ops 2
MW1 started the whole leveling up and unlocksing stuff craze (it wasn't in the previous CoD games, was it?), and was great because of that, though I didn't play it that much. MW2 was good, but was plagued with glitches people could exploit, and the maps were a mixed bag Grenade spam, grenade launcher spam, and rocket launcher spam was a huge issue, and made some matches just unbearable. Black Ops was great, it did some new things with unlocks and customization, had overall memorable and fun maps, and had very few glitches/exploits, and limited/balanced grenade use without making them useless. Black Ops 2 was like a revisiting of BO1, with good maps, some new ideas that kept things fresh, and good balance and low number of glitches. It did have some pretty annoying lag compensation though.
I heard MW3 was an unbalanced mess with poor map design, and I'm hearing similar things about Ghosts. Apparently most of the maps are very drab in color scheme (even compared to the usual CoD design), have way too many intersecting pathway (leading to maps that don't flow well), and there's 1 or 2 weapons people are sticking to because of how powerful they are. The drastic change to the prestige system gives you 5 characters to build up simultaneously, and you have to level them up and unlock their weapons/equipment separately, which just sounds annoying. Then there's the tech issues, like unreasonable performance on PC, rumors of framerate hiccups on PS4, and just the overall poor visuals of the game. I'm really hoping this is just a result of Infinity Ward's downfall, and Treyarch takes up the slack next year. As much shit as Call of Duty gets (and mostly deserves), the games are still fun (in MP at least). I sort of see them as Quake with real gun skins; fast paced, frenetic deathmatch where you can just jump in and kill some people. Though with a bit less skill required (aiming and weapon choice don't matter as much in CoD, but map navigation does).