Hey this is the best deck for modified its undefeated 15-0. Here it is its called...

The Amazing Deck

16 pokemon
3 rocket zapdos
3 cleffa
1 si mew [great against single color deck] tech]
1 brocks mankey [tech]
3 si onix
3 steelix
2 igglybuff

energy 17
9 electric
4 steel
4 rainbow

27 trainers
4 elm
4 gold berry
3 baloon berry
3 focus band
4 trap
4 sneak
3 nightly garbage run
2 mistys wrath

Alrighty, well...I don't think this is the absolute best deck you could possibly play in Modified. =/ I will help you tweak it so that you have a more diversified game plan. Lets get started...

First we have the Pokemon: Add in the 4th SI Onix. Drop the Brock's Mankey, and the SI Mew. I don't like the whole "attach my Rainbow energy to Mew, and leave him with 20 HP so he can be KOed next turn." The best target for Mew are babies, and the two most popular Babies are both Colorless (Cleffa and Igglybuff), so I don't like the whole combo. Drop the Igglybuff for Magby. Overall, Slowking's popularity has gone down a lot. Magby has the upper hand against Pokemon like Giovanni's Machamp, and Erika's Victreebel. Magby also can deal damage if he gets Mean Looked by Murkrow, and can't be hurt by Pichu like Igglybuff can. Alright, some Pokemon have resistance to Electric, all of which are weak to Grass. So lets get 1 Erika's Bellsprout. And also 1 Porygon. Porygon is great for changing anything with a weakness to something to benefit you. Both Bellsprout and Porygon make excellent tecH.

Next we have the Energy: Alright, drop the 4 Rainbow, and add in 2 more Electric, and 2 Darkness energy. The Darkness Energy will allow you to get some FTK against Babies, Totodiles, and other things.

Finally, we have the Trainers: Drop the 4 RSA, and the 4 Traps. With all of the Cleffa floating around you will find that hand disruption is nearly ineffective against most decks. Add another Misty's Wrath for some extra card drawing. Add in 3 Chaos Gym for anti-Gatr. And then add in 3 Healing Fields. The Healing Fields will break your Chaos Gyms in case you need Trainers. And it will also help you if you want more healing for your Steelix/Rocket's Zapdos. Drop a NGR, and add in 2 Energy Charge so you can get back Metal/Dark. So here is the final deck:

The Almighty Deck...Gymbo Style
Pokemon: 17
4 Onix (SI)
3 Steelix
3 Rocket's Zapdos
3 Cleffa
2 Magby
1 Erika's Bellsprout
1 Porygon (TR)

Energy: 17
11 Electric
4 Metal
2 Darkness

Trainers: 26
4 Professor Elm
4 Gold Berry
3 Focus Band
3 Balloon Berry
3 Misty's Wrath
3 Chaos Gym
3 Healing Fields
2 Nightly Garbage Run
2 Energy Charge

*Pojo.com Deck Mechanic
*Former Top 100 Global DCI Player
*Member of Team Great Lakes and Team NEO
*Creator of:
      ~The Espeon Project
      ~Scrambled Eggs
*Co-Creator of:
      ~Flaming Snausages