I use Jellicent to come in on Keldeo; if it uses Hydro Pump I'm free to Toxic / Recover. Believe it or not, HP Ghost 2HKOes after one Calm Mind, so you basically give Keldeo 2-3 turns of Toxic damage. Regarding Manaphy, of course it is low on the usage stats because it practically only functions in rain. Its ability to recover completely in one turn is wasted in any other conditions.
harsha, regarding Genesect, Choice Scarf is easily worked around, and a Genesect can't defeat all of those Pokemon without switching in over and over. Entry hazards wear it down quickly enough to allow for unboosted Excadrill or Scarf Landorus to KO with Earthquake.
Pokemon Crystal breeds:
Porygon (15/15/15/15/11) (full stat exp.) Lv 100
Alakazam (15/x/15/15/15) (full stat exp.) Lv 100
Starmie(15/x/15/15/15) (full stat exp.) Lv 100