From experience, one of the most powerful decks I think are decks that can heal themselves.  Like Bulbasaur's Leech seed.  I was wrote down a list of all the pokemon that can healthemselves in an attack.  If you are going to make a deck like this, consider these cards:  ((I am doing Base Set - Team Rocket - Promos))

 

Clefairy - Metronoming a healing attack.  ((Not really a 100% heal but I had to put it in, because it is possible))

Kadabra - Recover Attack (Remove all damage counters for discarding 1 psy energy)

Bulbasaur - Leech Seed (Heals by 10 hp, no matter how much damage is done.  Except 0)

Starmie - Recover Attack (Remove all damage counters for discarding 1 water energy)

Clefable - Metronoming a healing attack.

Vileplume - Pokemon Power: Heal (flip a coin, remove damage counter, once per turn)

Butterfree - Megadrain (Remove damage equal to half the damage done.  Base Damage is 40)

Exeggcute - Leech Seed (Heals by 10 hp, no matter how much damage is done.  Except 0)

Venonat - Leech Life (Remove damage equal to damage done.  Base Damage 10)

Ditto - Transforming then doing a healing attack/power

Kabutops - Remove damage equal to half the damage done (round up, base damage 40)

Golbat - Leech life (Remove damage equal to damage done.  Base Damage 20)

Slowpoke - Spacing out. Flip coin, if heads remove 1 damage counter

Tentacool - Power: Cowardice (Put the card into your deck)

Zubat - Leech Life (Remove damage equal to damage done, base 10)

Abra (TR) - Dissapear (Discard all energy from this card then shuffle back into deck)

Lv. 12 Promo Jigglypuff - First Aid. ((Remove 1 damage counter))

Lv. 64 Promo Venusaur - Megadrain (Remove damage equal to half the damage done.  Base Damage is 40)