Since I don't own the first two and Skyrim, but I do own 3 and 4, I guess I'll mainly talk about those.
Morrowind and Oblivion are awesome in my opinion. The downside to Oblivion is that the females look....male...ish...yeah, but other than that, I love it. The Elder Scrolls series is a real gem for Bethesda Softworks. I wonder who here agrees?