The 5 best are generally agreed to be:
In 5th Place: Electrode. It has decent level 1 stats, and has Sonic Booom, a ranged attack that deals 55 damage, which will OHKO almost all the Pokemon in Purity Forest
In 4th Place: Kecleon. It has awesome level 1 stats, and decent level 1 moves, and levels up really quickly
In 3rd Place: Alakazam. Excellent level 1 stats, and good level 1 moveset, including Teleport, which can get you out of tricky situations.
In 2nd Place: Mewtwo. Its level 1 stats are murderous, and it has a good level 1 moveset, and is probably the strongest.
In 1st Place: Mew! Its level 1 stats are alright, but it can learn every single TM, and there are heaps of TMs in Purity Forest, and it levels up really quickly.
Your choice. I either use Mew or Electrode. The first time, when I reached Celebi, I was using Electrode, but only because I couldn't get Mew.