Hello everyone can you tell me what is the best Pokemon to take to purity..

Any Pokemon will be fine at the Purity Forest..but since Celebi is at the very last floor.Bring a Fire Type so beating Celebi is not a problem. Item's are very vital..bring only the one's that has great effect on you for example. Apple's and Berries.

1.well the best pokemom I would say is either charizard(probably best level 1 moveset).Or alakazam hes defence is quite high for that level.
Im not sure what level is best to train on but if youve got some max elixers and a tight belt hang on the low foors as long as you can to train up for the higher floors

2.
Well the best items to take are

Max elixers
TightBelt
Weather band (very big sandstorms for any floors)
Reviver seeds
Cleanse orb(if your items like tight belt become ticky you need them cleaned)

Well I never needed this that mch but if youve never been there beforeit is best to come prepard.

The best Pokemon to bring is probably venasaur cos of his very fast leveling or alakazam cos he can teleport out of bad situations.

Ive never done western cave so I can't answer that 1

Ignore cyber_john3000's advice as since you don't need to fight Celebi, why on Earth would you need to bring along a Fire type to beat him?!? Celebi will not fight you, he'll join your team without any incident, it just wants to be on your team, so if you get to the 99th floor, you'll just get Celebi. Charizard is a good one to bring for the trip though.

Western Cave-definitely have a Tight Belt(not any food really in there), there's a lot of Reviver seeds, so that's not a big deal, but depending on your leader's level (Mewtwo is level 40, so your leader must be higher, but Mewtwo is a tough recruit, so you'll need to be over level 90), a Friend Bow might be helpful as I doubt you want to scale the 99 floors again if you don't need to! You also need a Pokemon with Surf or it in your inventory, and if you want to a key to get the Beauty Scarf on Floor 59.