Fallout 3. I know New Vegas has a better story and proper ADS. I know 4 is a markedly better handling experience. But 3 holds a special place in my heart
If you aren't already aware, check out Tale of Two Wastelands. Merges the Capital Wasteland of FO3 and the Mojave of FNV into one, keeping all the good bits Obsidian introduced with FNV.