In any modern 3D Fallout games I have to say fallout new vegas was the best.
Both in modding and vanilla gameplay.
Fallout: New Vegas took that great RPG/FPS mixture of a foundation Bethesda built - and basically went to town with it, content-wise. In FO:NV, we had much better writing; narrative; dialogue; actual roleplaying + decisions to make here than Fallout 3 ever dared. Also, the humor from Fallout 1+2 returned, which was missing for the most part in Fallout 3.
Pretty much, FO:NV (when it came out) only lacked the freshness + revolution that Fallout 3 delivered, since FO3 set the excellent standard here for all the future FO games to be built upon.
Fallout 4 is great, but a different kind of great. The combat + Quickloot here is just leagues above what FO3+FO:NV had going on. This stuff makes FO3+NV look+feel clunky-as-hell, TBH. Boston is an absolutely fantastic setting + Bethesda did a great job w/ making this a sandbox that I just want to keep on exploring. Nobody does open-world sandboxes like Bethesda, TBH - as they pretty much out-class everybody alone in this realm. I do wish there were more pre-built Towns/Cities, though - as they seemed to really take a backseat to the Build-A-Settlement mechanic.
I was disappointed as can be that pretty much they streamlined the hell out of it. RPG elements got streamlined - where they rolled everything pretty much into the Perk system. Decision-making got weaker - i.e. not many choices in most cases, TBH. Actual role-playing here got even smaller, where I began asking questions of "Ummmm, why are most options here often different shades of good (i.e. nice good or sarcastic good)? Where's the evil and neutral choices? Even FO3 did a better job at this!"
Regardless, I still got over 110 hours with Fallout 4 - and I think it's great; a different kind of great, mind you. FO4 seems to lean way more in the FPS/RPG hybrid (with more emphasis on FPS) than say the RPG/FPS hybrid that FO3+NV had going on there - and I think that seems to be why many people are not in love w/ FO4 like they were say with especially FO:NV.
Now, if we can just get Obsidian to do a new Fallout game w/ their typical Obsidian hallmarks (writing, narrative, story, dialogue, choices galore, dark humor) and Fallout 4's engine+combat, I'd certainly be very happy.