ok you get two votes, two for overall another for starter they can be from any generation and a reason would be nice

my vote is fore sceptile
reason
fast, and strong. he has low defence and hp but makes up for it easily with STAB and high spc attack giga drain plus add in sunny day and solar beam with energy ball and he does great.

over all my vote is for sceptile yet again i just love grass types since they are good against ground and have the usefull grass knot along with hp steal and restore moves which can make them last much longer than others, sure moonlight and roost are fine but have a set amount of healing and dont do any damage plus their result is almost always erased in the next if not same turn while hp steals moves of grass can get STAB and do damage while healing you as well as they can sometime heal more than the above stated
now take sceptile with giga drain at 75 base power
im not sure how all the boosts are calculated but its either
STAB+OVERGROW which each boost by 50% so stack that to a double boost and add in a life orb for another 30% and thats 130% damage boost which makes for 175 base damage or in rough calculation
75+50%=107.5+50%=153+30%= over 200 base damage and the life orb recoil is more than heal and thats all without taking into acount his deadly special attack

my second vote is to serperior for its speed and hidden ability and purely for that and the fact it has leaf storm but it would be better on sceptile to be honest, imagine him with contrary, a life orb, leaf storm and giga drain.
foe sends out pokemon
sceptile charges out
sceptile uses leaf storm and spc attack rises sharply
foe uses any random move and fails to know him out
sceptile uses giga drain/leaf storm and foe is knocked out
now rinse and repeat