Im going to be quite general and give you the top 10 most used in each tier. 1 is the most used, 10 is the 10th most used. *Note:* the order may be a little inaccurate but generally if it is wrong it's the same Pokemon with a slightly different order. These are stats from october so don't yell at me in the comments.

**Ubers**

1- Primal-groudon

2- Xerneas

3- Arceus

4- Darkrai

5- Mewtwo

6- Kyogre

7- Greninja

8- Ho-Oh

9- Yveltal

10- Salamence

**Overused**

1- Talonflame

2- Heatran

3- Scizor

4- Landorus-T

5- Rotom-wash

6- Garchomp

7- Charizard

8- Skarmory

9- Gengar

10- Ferrothorn

**Underused**

1- Salamence

2- Swampert

3- Donphan

4- Crobat

5- Hydreigon

6- Forretress

7- Beedrill

8- Nidoking

9- Chandelure

10- Empoleon

**Rarelyused**

1- Hitmonlee

2- Exploud

3- Alomomola

4- Flygon

5- Banette

6- Meloetta

7- Rhyperior

8- Sharpedo

9- Hitmontop

10- Tyrantrum

**Neverused**

1- Audino

2- Lanturn

3- Drapion

4- Magmortar

5- Golbat

6- Sandslash

7- Rotom-C

8- Registeel

9- Malamar

10- Gurdurr

*Note:* From experience I would also worry about sawk, scyther and mismagius.

**PU**

http://www.smogon.com/smog/issue37/introduction-to-pu

https://docs.google.com/document/d/1rk_LTrme-tsH4XNhl1SuAfsMEPfHYMz23ZZo3ySQyHY/edit?usp=sharing

I couldn't find any usage stats, but reading this will help a lot, consider Pokemon listed there threats in PU.

**Little Cup**

1- Mienfoo

2- Pawniard

3- Fletchling

4- Spritzee

5- Chinchou

6- Staryu

7- Drilbur

8- Diglett

9- Abra

10- Timburr

I hope this helps you in your future endeavors.

Source: http://stats.pokemon-online.eu/past-stats/october-2015/