I'm doing a playthrough of 3, New Vegas, and 4. Mostly focused on beating the main quests and the DLCs but I want to make sure I hit the best side quests on the way. I'm making my push to finish 3 right now and I've finished the DLC, Oasis, cannibals, vampires, ant quest, the other ant quest, and a bunch of stuff I'm probably forgetting.
Quests marked or unmarked from any of the games are welcome, and if any fans of 1 or 2 want to share I'd be glad to hear about those too.
I did the Mechanist quest line in 3 for the first time recently, not sure how I missed it on my first playthrough in 2008. Like with the Silver Shroud quests in 4, I just love the over the top retro comic book vides.
I have my problems with Fallout 3, but that mission and "You Gotta Shoot Them in the Head" (which gets bonus points for having a secret outcome that's not explicitly mentioned and comes up organically and involves breaking into a heavily armed military compound) are some of the best quests in the series.
Fallout 2 is basically a mediocre main story existing as an excuse for you to wander the desert and stumble onto all the phenomenal side quests. The murder investigation for the Wright crime family might be my favourite, but all the intrigue in New Reno is so good. And that's not even getting into the crazy Scientologists and their space ship, a shotgun wedding, uncovering the origins of Jet, becoming a pornstar...
Fallout 2 is one of my favourite games of all time, but it is OLD. By that I don't mean its graphics are ugly but rather the design philosophy is old school (some might say outdated). The stats and skills and perks are not balanced (some really suck and it's very possible to gimp yourself). There are some unintuitive puzzles and interactions you might need a guide for. Most of all though it's HARD and unforgiving. Save constantly and be ready to reload. The start sucks. Be prepared to hate the first hour(s), when you have to walk through the desert with just a stick and fight off brutal random encounters.
It eventually opens up, especially when you get access to a certain Highwayman, and becomes fantastic. As for Fallout 1 or 2, it's mostly a matter of preference. 1 is much shorter, smaller in scope, but also has a slightly more serious tone. It leans a little more into the whole harshness of the wasteland thing. 2 is much bigger, with so much more you can do in it, but it's also goofier. It leans more into the Fallout humour, pop culture references and sometimes just absurdity.
Also, look for some mods. I always recommend Restoration Project, Updated at a minimum, it's a combination unofficial patch and cut content restoration that is very unintrusive. I have a couple other suggestions if you're not against a sprinkle of mods. Most are on Fallout 2 Nexus I think. Mainly convenience stuff. There is a FANTASTIC Talking Heads mod that perfectly captures the original artstyle, but it's obviously not vanilla. Same for the fan made voice acting mod.
Lastly, you can't talk your way out of the final fight in Fallout 2. I just want to save you from frustration, since you can talk your way out of most other situations. Don't brick your save by investing nothing into combat stats.
Phew, sorry for the rant. I'm really passionate about this game.
In New Vegas, completing Arcade Gannon's companion sidequest gives you a suit of Enclave advanced power armor, and a squad of Enclave troops with a vertibird will back you up at Hoover Dam. It's virtually impossible to trigger this quest by accident.
I have, and the only thing I'm not trying is disabling multi-monitor support.... but it still crashes to desktop. suspect that's the culprit but I'll be damned (really, can't) if I have to turn off multiple displays lol
For Fallout 4, in no particular order:
Troubled Waters
Pickman's Gift
Here There be Monsters
The Big Dig
Last Voyage of the U.S.S Constitution
Hole in the Wall
Silver Shroud
Cabot House
Spectacle Island - while not a quest, this is a good settlement area if you're into settlement building
A bit off topic, but if you have access to mods, there are some great quest mods for Fallout 4. I didn't do much with modding for 3 or NV.