I've seen the ups and downs of each of the 6 colors, as well as colorless,
and decided to express my mews....er, views on them. (Little poke-humor
there...)

Fire- 
UPS: A lot of the creatures have good attacks, as well as decent attacks
that don't require fire energy. Take Ponyta for instance. It can deal 20
damage with 2 energy of any sort, or 30 for 2 fire energy. Not too shabby.
DOWNS: Obviously the major downside is with the better attacks requiring
energy discarding. This forces you to load your deck with Energy Retrievals
and a lot more fire energy, which really squeezes out anything else.

Water-
UPS: It has many good evolved Pokemon. Gyarados, Poliwrath, and the
almighty Blastoise for example. They have extremely powerful attacks (A
gyarados doing 100 damage to a Charizard in one hit. YEOW!)  that can
flatten almost anything. And as for the so-called "Outcasts" of
Staryu/Starmie, they're not too bad either. Staryu can do 20 damage for 1
water energy! BARGAIN! And for discarding ONE measily Water energy, you can
remove all damage from Starmie! Better deal than Pokemon Center!
DOWNS: Some of the basic pokemon tend to suck. Poliwag can't really do
much, although with enough energy Water Gun can do 30 damage, which is
nice...if it lives that long. Also, Magikarp...yeesh, I'm not even gonna
TRY. 30 HP? 10 damage attack? And an attack that does NOTHING when it's at
full energy and only does 20 when it's about to die? No thank you! Gimme
Gyarados!

Electric-
UPS: Great attacks. Zapdos's attack of 100 damage is D-day to ANY water
pokemon.
Raichu's Agility is like 2 Thunderbolts AND a 50% chance of Barrier! Mewtwo
eat your heart out. Also, if you're in a squeeze for energy and have a lead
in the Prizes area, you can use Electrode's power to get 2 energy of any
kind you want! Good deal!
DOWNS: So many of the attacks wind up hurting the pokemon itself.
Magneton's Self-Destruct is a last-resort move only, as he'll blown himself
to kingdom come unless he's got about 3 Defenders locked on to himself.

Grass-
UPS: The king of status affliction. Koffing's attack may seem overpriced,
but since it will either Poison OR Confuse the opponent, with no chance of
failure, you're bound to do a lot more than 10 damage. Also, Venusaur is an
absolute massacre-maker. And Beedrill and Butterfree are also good.
DOWNS: Some of the basic pokemon tend to bite. Nidoran M has the problem of
an attack that only has a 50% chance of doing anything. And even the stage
1 pokemon can bite. Metapod and Kakuna have low HP and overpriced defensive
abilities for their HP levels in my opinion.

Psychic-
UPS: Jynx's moves can slaughter just about anything. Mewtwo will deck just
about ANYTHING with Barrier, preventing it from biting the dust for a long
time. And Alakazam's power to redirect damage is just plain SUH-WEET!
DOWNS: The basic pokemon can just plain BITE. Abra. 30 HP. 10 damage. Joy.
:-b
Also, the stage 2 Pokemon could use some work. Sure, Mewtwo can survive as
long as Barrier's up, but a few Energy Removals and it's hasta la vista
kitty.

Fighting-
UPS: Powerful attacks for little energy. 1 fighting energy for 20 damage
makes Machop a great basic pokemon. Hitmonchan will pummel anything your
opponent's got into oblivion, and with a few defenders can stand up to even
the great Fire Spin. Not so hot now, eh Charizard?
DOWNS: Blastoise/Venusaur chow. Not fun.

Colorless-
UPS: Most of them are strong against those annoying Psychic pokemon.
Porygon and Farfetch'd can really mess up your opponent by screwing up
their weaknesses and dealing a whopping 30 damage for 3 energy of any kind.
Fun! And Raticate'll bring the house down with Super Fang. Let's see, Mr.
sleeping Charizard. 60 damage first turn. Now I'll pull out a water pokemon
that can do 30 damage and slaughter ya.
Whoops, almost forgot the 2 killers in the group: Clefairy and Chansey.
Chansey can stall the game for a while, and if you're on your last prize,
win the game with Double-Edge. Clefairy is nasty against a damaged pokemon
with strong attacks. Metronome the attack and blast the sucker into the
twilight zone.
DOWNS: They just don't have enough HP to survive a lot of attacks, and some
of their attacks are rather weak. Dratini. 40 HP, 10 damage attack. Pidgey
does just as well AND has an included Gust of Wind!

All in all, I'd say no one of them is truly superior to ALL the others.
They each have their strengths and flaws that, if taken into consideration,
can be used to fine-tune your deck to be almost unstoppable!

Xaq Shenor