First two are cRPG's. Isometric perspective, turnbased combat, character skill a centered.
3 and New Vegas are first person shooter hybrids where 3 is more focused on random running around finding random things and NV is focused on developing a storyline and outcome through the choices you make; but on a technical level the games work largely the same.
Make your pick.
Personally I'd take the first two over the latter two any day, but your mileage may vary. I'm more entertained by interesting RPG mechanics and how I need to play with them in an interesting world with an interesting storyline, than sloppy first person shooting (even if it may get the story part right like NV largely does).