using emerald for gains reduction can be useful, since tanking an emerald makes it so much harder to landchain anything reliably. it is reliant on being able to stick the emerald on target, and being able to pick the right targets to attack (i.e., not just chain attackers). most ghettos shouldn't touch emerald with a ten foot pole, but then ghetto wars are very lacking in anything resembling strategy.
would still say ruby or gold are the least likely to **** up, but that is very contingent on setup. ruby is pretty useful for kingdoms that counter-wave, as it becomes easier to hit in nw-range safely - particularly useful for halflings and faeries that attack on the opening wave.