Let's be clear about "octane". In the USA you use the 'AKI' (anti-knock index) or (R+M)/2 octane number (the average of the RON and MON). Here in Europe (and pretty much everywhere else) we use RON. For that reason our "octane number" is always higher than American ones, for similar grades of fuel.
I use 'regular' unleaded gasoline, which is 95 RON. We get 'premium' options from various suppliers, usually in the 97/98 RON area. Remember though that octane number is not a quality index. However, the premium fuels with higher octane numbers often also have extra additives for ignition improvement and detergency, so tend to be better.
Remember also that unless your engine can take advantage of a higher octane number (by dynamic ignition advance, or if you have manually advanced the ignition timing) then it won't offer much benefit (aside from the potential to clean up fuel system deposits). Also, an older engine may benefit if it is coked up - the carbon deposits in the combustion chamber have two effects on the combustion: (1) they act as 'glow plugs' and can pre-ignite the charge and (2) they reduce the combustion chamber volume, this increasing the compression ratio. Both can cause pre-ignition which a higher octane number fuel will resist better.