1 = Nice start to the franchise, but it's way too short with bugged quests and useless skills galore. The title screen introduction is a great example of show don't tell in how it establishes the setting and backstory.
2 = 1 but better! More depth to quests and ending slides, more weapons(Bozar! Machine gun sniper rifle!), more areas, companions that aren't useless in the lategame, the protagonist is more expressive in conversations, only one major quest IIRC is bugged where you can't get the good ending!
Never played tactics.
3 = Oblivion with guns, as the saying goes. A less obnoxious level scaling system, horrendous writing, a goddamned level cap, and worst of all, no ending slides. Liked the Bethesda exploration though, and the quests actually tend to have multiple ways you can approach them in Fallout fashion.
NV = This masterpiece, despite being rushed and partially complete, is the zenith of WRPGs. It takes the design philosophy of 1 and 2 and beautifully translates it to 3D. The wonderful myriad labyrinth of choices, outcomes, factions, companion sidequests, nearly every side quest in the game affecting the ending, and ending slide permutations means you will probably get a different ending every time you play. Add fantastic writing and world building and we have a masterpiece that could have been even better if Obsidian had more time to work! Even the level cap is not really a problem if you start with the right trait since I just barely hit it after completing the final DLC!
4 = I heard that, partway through development, half the team got reassigned to work on 76, and it shows. Half baked ideas like the combat zone, the robot race, and the raider gangs' interactions everywhere you go! I did, however, adore the settlement building aspect despite being rather shallow, since it gives a feeling of progress as opposed to "watch the numbers go up". The exploration energizes rather well with the settlement building aspect since you now want every single scrap of stuff Bethesda loves to fill there worlds with. Also Nick Valentine is one of the best companions in the entire series. And no level cap! Hooray! And still no ending slides! Boo! Far Harbor is noticeably better written then the rest of the game + DLC.
76 = One of the scariest games ever made, and it's not even a horror game! This game attacks you, the player, on a meta level. Unlike Eternal Darkness and the like, this one shows no restraint whatsoever. What will happen next? Will you receive a cheap plastic bag instead of the canvas they showed to the media? Maybe the 50 GB beta will refuse to uninstall unless you buy it? Perhaps customer support tickets containing your personal information will be publicly accessible due to horrendous web code! I bet hackers will find ways to steal your inventory including microtransaction-exclusive items! Maybe the special edition helmet will start molding! Class action lawsuits got filled got filed in response to no refunds!
The worst part is that it never ends and could happen at any time! You are never safe!
That said, literally every single game in the series requires an unofficial patch for the best experience.
NV>2>1>=4>3>76