1/ pro Oblivion, because they are driven by people who only played Oblivion
2/ Pro MW, because people have played MW before playing Oblivion
3/ some are pro DF, because some started the serie with DF
...
How I see it, from the point of view of those who have experienced the whole serie, there was a great improvement from Arena to DF, which made the serie a special one. DF was revolutionary, compared to CRPG of its time. Then, MW drove the serie away from the generic Medfan crowd with its unique background, but at the expanse of the role playing quality and gameplay richness. Then, Arrived Oblivion, with an even more mainstreamed gameplay, but with also a return to a very generic Medfan background and a dull plot.
The serie evolution, from the gameplay mechanics and story point of view is quite hidden by the technical improvements in the graphic, animation and sound chapters. But, considering technical improvements as the norm in the computer games world, and the other games released at the same time as TES ones, it is not a key feature. Removing the technical elements from the anolysis, I think the apogy of TES was between DF and MW.