Some great advice here on reverse types, so I will not add to it. My advice: there was a wide range of quality in the engraving of Aurelius' bust, look for one with good style.
When it comes to the denarii of M. Aurelius I tend to split them into 'younger' and 'older' head types. Aesthetically I prefer the younger head type (which I am guessing predominates on his coins as young Caesar), with scant beard as shown in this denarius. Otherwise the older head types bear many interesting reverses, as for example this one with the god Mars: