RE: worst stage 2
Sceptile? I think it is good. The 60 damage attack is swift: resistance and metal energy won't be a problem. Its poké-body is good, and you can use boost energy...
The worse stage 2 I think is a tie between Butterfree Jungle,Kabutops fossil and Jumpluff neo 1. The first 2 used a lot o energy in a time ER and SER was common and they had only 70 and 60 HP. Jumpluff from neo 1 strongest attacked caused only 20 damage. If opponent had resistance, no damage. If it had 100 HP, 5 turns to kill it.
From the modified ones (RS on), I think it is that POP 1 Swampert, because, its attack is weak for a Stage 2 and slow because uses two water energy, it has only 100 HP and 2 for retreat. It could never beat a meganium which has resistance and that annoying recovery power.