Arena: Awesome if you regard when it came out Daggerfall: Awesome Morrowind: Truely Awesome Oblivion: Awesome if you never played one of the former es and don't want original stuff.
I am not sure about Arena and Daggerfall but I would describe the last 3 games by Bethesda (Morrowind, Oblivion, and Fallout 3) as the Good, the Bad, and the Ugly.
I remember a point, a couple years ago, where I was playing it every day. It's truly a unique gem and the only game that does what Arena tries to do better, is Daggerfall. Arena most certainly is not forgettable.