Basically, you run and rerun The Stony Field, The Dark Wood, and The Black Marsh looking for 'Gem' Shrines. On SP, you can engage in Gem Shrine Farming. Oh, btw, for gems, see option 1 up there. For reasons that I don't recall right now, her rune drop is best in a solo game. Always (.after the first 'quest' drop) run the Countess solo on bnet or on /players1 in SP. In Hell, though, she's really not that great as she can't drop the highest runes. The Countess is a good way to get low to mid runes throughout Normal, Nightmare, and Hell.You always know exactly where the council will be when you join a game. One of the best features, though, is that the map is static. The council in Travincal have a decent chance to drop a lot of good items (runes, charms, jewelry, and facets, for example). It has a higher area level, so it can drop higher runes than LK, but it only has at most 2 such chests. This area has up to 6 'super chests,' which have a decent chance to drop high(er) runes.

The highest rune I've found in SP is Cham in The Glacial Trail (which has area level 83). The highest rune I've ever found on bnet was Sur in the WSK. edit: champions and elites in those areas can drop any item in the game). The Pit in Act 1, The Ancient Tunnels in Act 2, The River of Flame in Act 4, the Worldstone Keep in Act 5) as they can drop any rune (or nearly any, my memory is fuzzy.

Really, just play the character you like most in the highest level area that you can clear quickly. To be fair, though, the OP didn't specify higher runes. The Countess is not the best source of higher runes.

The one with the lowest rating is correct. Some of the answers here aren't very good.

Diablo 2 countess farming bot