I had to go with TES, and rather dislike having to make that choice.

ROA, Ultima, M&M, and Gold Box I haven't played. NWN, FO, and Gothic I detest to various levels. BG rates as just tolerable. And while I really like Wizardry, I only consider the last three entries to be RPGs — the first five are dungeon crawlers that slowly picked up RPG elements. So it's hard to rate Wizardry high as an "RPG Series" Fun, yes, but overall lacking in elements I consider essential for the "RPG" tag.

That leaves TES as the one series I can credit as an entire series. (Yeah yeah, I know, <insert random rude oblivion comments>.)

If we were rating individual games from the different series, my response would be vastly different.

I did finally get a copy of the Ultima collection (sadly, no Worlds of Ultima included), and I've been trying M&M on and off. In both cases, I'm planning to go through them in order. Now if only I could find a reasonably priced copy of the RoA series. I don't like/trust downloading games.