Chlorophyll

In-Game Text: Boosts the Pokémon’s Speed in sunshine.

In-Depth Description: The Pokémon’s Speed stat is doubled when either Sunny Day or the ability Drought are active on the battlefield.

This is a phenomenal ability to use on a Sunny Day-themed team, alongside Drought Ninetails, Mega Charizard-Y, or Groudon. However, unlike Swift Swim users, which get the speed boost in addition to STAB on their powerful Water-type attacks, all users of Chlorophyll are Grass-type users, which makes them even more vulnerable to Fire-type attackers. Regardless, the Speed boost provided by Chlorophyll is very welcome to the majority of its users, and allows them a second lease on life in most cases.

One obvious trick is to use a no-charge Solarbeam in conjunction with Chlorophyll, and this is a great strategy for Grass-type Pokémon with high Special Attack stats, such as Exeggutor and Vileplume. Venusaur in particular rose to glory with help from a buffed Growth (now providing a +2 boost to both Attack and Special Attack in the sun), and was one of the most devastating users of Chlorophyll in the Generation V Overused tier. Shiftry is also a particularly nasty Sun sweeper in Ubers when used alongside Groudon, with access to STAB Dark-type attacks that hit the many Psychic-type Pokémon in the tier super-effectively. Furthermore, it can boost either its Attack or Special Attack with Swords Dance or Nasty Plot, respectively, making it difficult to switch into if you aren’t using Kyogre or Politoed to get rid of the sunlight, as Shiftry outspeeds both of them naturally even without help from the sun.

As of Generation VI, the sunlight brought with Drought is no longer permanent and lasts five turns (eight if using Heat Rock as a hold item), and the popularity of weather tactics has diminished slightly. However, it is still a very viable tactic with the proper support.

Pokémon With This Ability Available
Oddish
Gloom
Vileplume
Bellsprout
Weepinbell
Victreebel
Exeggcute
Exeggutor
Tangela
Bellossum
Hoppip
Skiploom
Jumpluff
Sunkern
Sunflora
Seedot
Nuzleaf
Shiftry
Tropius
Cherubi
Tangrowth
Sewaddle
Swadloon
Leavanny
Petilil
Lilligant
Maractus
Deerling
Sawsbuck
Bulbasaur (Hidden Ability)
Ivysaur (Hidden Ability)
Venusaur (Hidden Ability)
Leafeon (Hidden Ability)
Cottonee (Hidden Ability)
Whimsicott (Hidden Ability)