Its not even a debate, Elder Scrolls V has been whats got my mouth foaming for years. We know they are basicly working on ESV anyway, but honestly id be a bit disapointed if they in a change of fate started working on Fallout. I love Fallout dont get me wrong, but Oblivion is my favorite game of all time. No other game has ever made me sink so much time into it. I know nearly every quest, ive got every piece of DLC, and I put up with the many bugs on the XBOX for it. It is a one of a kind game not matched by anything else. I mean, and open ended, action heavy, western, medieval, fantasy, RPG that features so much diversity and amazing lore, has easily launched it to the top of my list. Morrowind is up there and it may be controversal, but I dont think it can hold a candle to Oblivion. I personally found things to get too repetative, and when things got pretty sweet on my PC version of it, it would usually crash all the time. Fallout 4 will come after ESV, and I wouldnt have it any other way. I know the big 5 will have me sinking thousands of hours into it.
EDIT: and im sure taking what they've learned from all there past games, what worked and what didnt, will make ESV truely a masterpiece.