everybody on here is gonna throw out something different.
Where are you skiing?
What are you currently skiing?
How many days a year do you ski?
Height/Weight/Ability are keys, as well.
If you're on the EC, then your definition of "All Mountain" will vary greatly from say somebody in Colo/Utah/Tahoe/PNW.
Some will claim that a narrow (74-78mm) waisted ski makes a good All Mountain ride. Others will claim that a mid-fat (80-87mm) will make a good All Mountain ride. And still others will claim that a sorta fat (90-100mm) ski will make a good All Mountain ride.
Me, I have 3 active skis in my quiver, all of which are deemed All Mountain, in a manner of speaking, by different folks. I ride a 78mm waisted 180cm long AK No Ka Oi, it works well in variable conditions providing that it's not either too icy or too deep on the powder. I also have a 87mm waisted 177cm long Karma. It rips in the bumps and turns on a dime and works well in light powder. Finally I have a 94mm waisted 177cm long Mantra that works pretty good in just about anything. I've ridden it on hardpack and have to edge hard, but it grips, I've ridden it on groomers and it rips, and I've ridden it in knee deep + powder and it floats.
So, it all depends on the conditions, your abilities, and most importantly where you plan to ski/ride the most.