1) stability + compatibility
2) how well it can be applied to existing savegames,
3) that it "feels" natural and realistic!
4) that it stays balanced throughout the game
1) They should all be okay there.
2) No idea. It is not something I would even try. But then, I tend to be overly cautious and meticulous, when it comes to modding. I do not expect others to be, if that matters.
3) No idea what
you mean by that. But from the "sales pitch" (not to mention name!)
Realistic Leveling would have to be high on the list, surely.
4) No idea what
you mean by that, again. "Balanced" is a bit of a funny thing to begin with, in Oblivion. Mods generally make the exact
variety (and perhaps degree, in some cases) of funny a little different, but regardless, Oblivion remains quirky and somewhat lacking, particularly at very high levels.
So, as usual, it comes down to personal preference. No matter how many people tell you "this one", or "that one", you will inevitably have to try one or more, and see what
you like best!
If it was up to me, I would tell you
nGCD + Progress, or
Oblivion XP, depending what kind of game you want to play. But there is nothing wrong with other options (e.g., Realistic Leveling).
As you can already see, there are many, many ways of approaching character leveling, in Oblivion. None "perfect", but some
superb. Plenty more, if none on your current list works out...