I want to say do not listen to people who have only grown up with Black Ops 1 and higher.
I have played every single COD and own all but one. I can tell you that as the games progress the quality declines. Here are the best three options:
Modern Warfare 1 (COD 4): this game is a legend, and it makes me sad that no kids today even know about it. They are all "MW3 yay" and never even considered or thought about 1. This game was the prime of COD, and what makes it great is the maps and simplicity. The campaign was great and it was the first ever modern Call of Duty. It is better than MW2 because it is a lot less cheap I tell you and all the hackers are gone from this game now online. People still play online.
World At War (COD 5): This game is very close to WWII and has great maps and introduces Nazi Zombies (when they were actually called Nazi Zombies). From large maps to small maps and a stunning campaign, you almost want to sign up for the next war in your country.
COD 3: There is something so nostalgic about it. If you are looking for a totally fantastic campaign and a multiplayer where you can drive motorized vehicles, the only game you can, and tons of tanks and jeeps, go for it! The music is beautiful too.