Notes:

http://www.smogon.com/forums/threads/smogon-usage-based-tier-update-may-2015.3537414/

Tornadus-Therian moved from BL to OU
Scrafty moved from UU to RU
Gallade moved from OU to UU
Kingdra moved from UU to RU
Mandibuzz moved from OU to UU
Flygon moved from UU to RU
Alakazam moved from UU to OU
Haunter moved from PU to NU
Heliolisk moved from RU to UU
Dragalge moved from BL2 to UU
Mamoswine moved from OU to UU
Hippowdon moved from UU to OU
Fletchinder moved from RU to NU
Sigilyph moved from BL3 to RU
Slurpuff moved from RU to UU
Camerupt moved from NU to RU
Magneton moved from RU to NU
Abomasnow moved from RU to UU
Doublade moved from RU to UU
Cresselia moved from RU to UU
Slowking moved from RU to UU
Noivern moved from UU to RU
Tauros moved from BL4 to NU
Kabutops moved from RU to NU
Seismitoad moved from NU to RU
Prinplup moved from PU to NU
Whimsicott moved from RU to UU
Pinsir moved from RU to NU