For "well known standard range" whiskies, two that immediately come to mind are:
Macallan 18yo vs 12yo
Talisker 18yo vs 12yo
And - not quite the same, but I'll put down Old Pulteny 17yo vs 12yo as well - I'm not really a fan at all of the 12yo, but the 17yo is absolutely awesome.
However, I think you have to be careful about the old adage of "age = quality". This is definitely just not true in a lot of the cases with whisky. One of my favourite distilleries is BenRiach: the 12, 16 and 20yo all kind of share the same flavour profile. I find the 12yo a bit young, the 20yo perhaps a bit too austere, but the 16yo just hits the money of enough of a brash character of the 12yo with just the right amount of mellowing from the 20yo.
Caol ila 12 vs 18yo is another example: the 18yo is definitely more mature and has mellowed, but the 12yo definitely has more character. Which is better? I think it really depends upon one's mood at the time (or just pick up the DE or CS versions instead!!!
And then look at Glenfiddich: I think you'll see that most people really like the 15yo Solera - even better than the 18, 21 or even most of the standard 30yo bottles.
However, the best part is just trying them all
to find out which you like!