The worst is R.Zapdos ex?
For me is the opposite, at least for the legendary birds...
Darkness guard may save it from attacks that deal 100 damage and you may pick it from your deck using Holon mentor because it has 100 HP (can't do the same with zapdos ex, the non Rocket one). It does 30 damage to one of your own benched pokémon, not really good... but also causes 70 with 3 energy (1 darkness), maybe in turn 2 if you used plasma last turn. 30 damage in bench is "healable" and plasma's cost may be payed with darkness energy, recovering a discarded a lightning energy (TV, R.mission, Holon supporter to discard...). In RS-on, he is interesting in Team aqua's manectric/lanturn deck (treansfering 2 [L] to Zapdos is easy, the same for attaching darkness energy), specially because he is pure darkness, so, no resistance to lightning applyed.
The most rare where I play pokémon is R Entei ex (only one person i know had it). The most common here is R Sneasel ex, because of Dragonite-Electrode deck.
It's possible that Nintendo printed more copies of R Scyther ex, in relation to other ex in TRR set, because R. Scyther is a basic ex that evolves, but this only they know...