Fallout 4 had the best graphics and shooting so far. The weapon customization was a great system, if poorly executed. Armor pieces instead of single outfits was also a nice change. I enjoyed the power armor system, even if there were some lore rewrites made to have it happen. Settlements were okay, I guess. They felt pointless after a while and I just stopped caring. Graphically and mechanically, the game works, I think. It's just not a great RPG like you would expect from Fallout.
The lack of RPG elements (RIP skill points, skill checks, character builds, etc) and the simplification of the game really hurt it. A voiced protagonist was a mistake too, because it limited the options the player had in dealing with NPCs. 4 choices (read: 1 rude yes, 1 ass-kissing yes, 1 neutral yes, 1 tell me more also yes) was an awful system. The lack of branching dialogue was also very bad. Removing item durability made the game feel too bland. There was less tension when you had infinite weightless ammo, no hunger/thirst, infinite money and unbreakable gear. It feels like Bethesda only wanted players to play through their game once, which is the opposite of how I felt in 3/NV.
The world was very good to look at, but it felt inconsistent with the story they were trying to sell. The bombs could've dropped yesterday for all the mess and loot laying everywhere. Fallout 3 has the same issue. 200 years have passed. There should not be locked prewar safes. After 200 years, nothing useful should be just laying around.
New Vegas is my favorite of the series. The new systems it had over F3 (skill/stat checks in dialogue, cybernetics, good DLC, weapon modification, branching quests) were excellent additions to the RPG. The gunplay was satisfying and a step up from 3. However, it was still very dated and somewhat janky.
The map was a little empty, but unlike 3 and 4 that matched what Obsidian was going for. It's a desert. The bombs didn't fall here. It's old, and it's empty. If anything, seeing people actually building things was a welcome sight in New Vegas. The casinos on the strip were a joy to walk through because they felt starkly different from the outside world.
NV's main story is average, IMO. The game's side quests are the real meat of the experience, and their multiple endings and faction attachments mean that you will need to do multiple playthroughs to see the whole game. I still haven't played a proper legion character all the way through. Several hundred hours and multiple years spent playing the game and I still haven't seen all of it yet.
My perfect Fallout game would be a game with the size, complexity, and RPG elements of New Vegas with the shooting, building, and crafting systems from Fallout 4.