I haven't played this franchise since COD4: Modern Warfare. I know it gets a lot of flack since then, but has there been any considerable improvement in the series since then? Like...would it be worthwhile to try Modern Warfare 3?
I would recommend Black Ops over any Modern Warfare game. It has the best campaign story, and multiplayer wise it is the most balanced COD game you will ever find. Treyarch did a spectacular job making the perks and weapons very balanced, in fact some would argue they went too far with that because they made it so snipers can't scope in at 0.2 seconds and kill someone at close range with a single shot. The so called "quick scoping" community hates the game for that reason, but realistically sniper rifles are supposed to be long range weapons only, so I think what they did makes the game far more realistic and balanced. They also made it much harder for people to cheat and boost in Black Ops, and they added in a way to report such people if you catch them doing it.
Cheaters and boosters is one thing which ruined MW2 for me. You would have two kids hiding in a remote corner of the map with a tactical insertion, and one would kill the other over and over again and after 25 kills they would get a nuke which would ruin the game for everyone. You would look at the leaderboard at the end of the game and there would be one player who has 25 or whatever kills and 0 deaths, and then another player with 0 kills and the equal number of deaths as the other players kills. You couldn't report them and Infinity Ward didn't give a crap that people did this. So that ruined the game for me. Black Ops fixed all that, and that's the COD I've been playing ever since.
As for MW3, they took the nuke out of that, so you don't have to worry about people boosting for those, but its extremely similar to MW2 to such an extent that people rightly criticize it for being little more than a glorified MW2 map pack. I suppose if you love MW2 then that's not such a bad thing, but since I didn't like MW2 I wasn't very impressed. I actually played it about a month ago and I was disgusted at how little has changed, and that they are charging people $60 for this patch.