But should we take the older games mre seriously? Or the newer ones?
Both should be 'taken seriously', but if you're wanting to know which is more to scale then go with Daggerfall - think real life. It has nothing to do with lore, its just game mechanics.
I mean should we take the newer games (OB MW) or the older games (DG Arena) more seriously in lore terms.
Not that they don't count but maybe they're updated and changed things, and some people don't even consider Oblivion to be canon...
Each new game updates and changes things. Very little from Arena is still around apart from what's derived from the main quest. Most things from Daggerfall and Morrowind still hold true.
Nobody considers Oblivion to be non-canon, to do so is just ignorant, however certain things in Oblivion can be dismissed because well, they svck. Of course you can stand by those things if you want your game world to svck, but chances are you don't... Basically, if nothing from a newer game contradicts it, then its solid. If something from a newer game does contradict it, go with the cooler one...