I've seen the second poll countless times, it's always the same. Oblivion has always had the most players, with Morrowind following close behind with a hefty group of its own, then Daggerfall carving a smaller niche for itself, and only the smallest minority of people have ever played Arena. In terms of true popularity, who's to say. New gamers are made every day, the gamer population grows constantly, and as a result there will always be a growth in the playerbase over time. There is usually a correlation between time and video game sales. Sounds weird to consider how many people pick up sequels without playing the first one though. Mass Effect 2 sold more than Mass Effect 1, Assassin's Creed 2 sold more than Assassin's Creed 1, and so forth.
Interesting that you mention those two games. My wife and I bought ME1 on Xbox Live for 20 bucks a few months prior to ME2's release. We'd heard it was a good game, had seen ME2 trailers and decided to give 1 a shot prior to dropping the money for 2. We really enjoyed 1, so we bought two.
The same thing happened with AC2. We'd seen some trailers, heard good things, decided to pick up 1 used from Gamestop. Really liked it, though hated certain aspects, and then were thrilled when 2 improved the hell out of the game.
So, our purchase of ME1 would have registered in sales, our purchase of AC1 would not have (being used). Just an example of how the sales for a sequel can far surpass the sales of an original. If we'd decided to get ME1 at something other than an ungodly hour, we would have purchased it used as well, and again, the sale would have gone unnoticed by the publisher, though the sale of the sequel would have been added to their numbers.