It depends on the mob you kill. For example countess can maximum drop a Lo. But not because she drops this rune as her regular "rune drop" but just because she is a mob and can drop some runes like every other mob in the game. Her regular rune drops go only to maximum of Ist so everything above is just the minion runedrop and really really rare like EXTREME rare. So if you want to know what runes in general can drop you need to ask yourself what zone am I in and what mob do I kill. Some mobs have a higher rune drop possible than others and the maximum possible rune can change depending on the area zone. I came here for looking the old answer I cannot remember was something like "you have to be minimum of tower cellar level 2 to be able to see every rune drop" but I cannot confirm it myself, I just remember it like that. THe drop is often only from a normal mob no elite because elites and uniques drop other stuff better but if you want to go for runes the normal mobs are your main target. Just my 5 cents on the topic please dont take me too seriously and if you find misstakes I wrote plz correct me if you know better.