Rainbow Scale, much like the Body of Charizard (Secret Wonders) makes an otherwise merely beautiful card into a highly interesting and tempting to exploit pokemon. The point that's quite interesting to consider with Rainbow Scale is that every attack that Garchomp uses does 40 more damage, as long as you play a specific color of basic energy onto Garchomp. Any attack, even if it's used via Memory Berry. Admittedly, at the time of writing, this leaves only Gible MT's Push Down and Garchomp MT's Dragon Fang open for damage boosts (as Gabite MT has no attacks that deal damage, per say), but this leaves possible combos open for later releases of Gabite and Gible.
Open-ended pokemon sure are fun to examine... Just look at what they bring to the table: Garchomp happens to have 0 retreat cost, which makes it highly flexible when it comes to your options for active pokemon. As already observed by others, the massive 130 HP makes it able to survive most hits and thus requires 100 damage as the minimum damage to one shot it. Notice the massive list of pokemon who can deal the 100 colorless damage to one shot Garchomp. A Blissey with 8 energy would be able to one shot it. That'd require at least 5 energy to start, plus a boost energy. Delcatty EX CG requires 10 energy to be in discard, which rarely happens at an early enough time to avoid being pummeled by Garchomp.
When you can deal 110 damage back to a Blissey with a mere 3 energy, it becomes evident why Garchomp is fearsome. The ability to singlehandedly rip apart decks such as Kricketune (which depend on low HP Stage 1s all sharing a common weakness) is incredible, when you also consider that it does this every turn for no additional cost. Every basic, stage 1 in the game (excluding Rayquaza EX d and Blissey), and all pre-Diamond/Pearl Stage 2 pokemon are one-shotted by this beast. The only possible way such an attack could be more devastating, would be for Rainbow Scale to add 20 more damage, so Garchomp would be dealing 130 damage (which would be a knockout on almost every pokemon, and thus completely broken).
Let's compare to other high damage dealing pokemon, shall we? Blissey has already been examined. Electivire DP is doing similar damage (120) for one more energy, plus discarding all lightning energy on itself. Magmortar LV.X is dealing less damage for one more energy, all have to be the same color, and two energies must be discarded (though there is an advantage in that it can hit the bench). Salamence EX PK is doing 30 more damage for one more energy and discarding the top five cards of the deck. Rampardos is dealing 10 less damage for three specifically colored energy (all fighting, which by the way happens to have no energy accelerating pokemon) and the drawback of also dealing 20 recoil damage to itself (though it does bypass resistance and buffer piece, which Garchomp can not)
All you have to do to gain this unparalleled straight damage advantage, is ensure you have one energy of the correct color attached, avoid using special energies, and remove all Battle Frontiers and Cessation Crystals from Play.
The Cessation Crystal Weakness is removed via Warp Point (which works out well, since Garchomp has free retreat, so sending a second Garchomp active solves the retreat cost issue). Battle Frontier is wiped out via Windstorm, or in a Holon Mentor-able Basic Pokemon whom accelerates your pokemon from the deck onto the bench: Tauros CG. 3 energy is attached all at once with use of Swampert EX CG, or 2 energy are attached at a time from the bench via Raichu MT and Exeggutor MT. You even have the option of using Gardevoir PK, whose damage placed on self can be healed with some luck and any Gabite (use Marvelous Shine and land tails). The correct energy being attached is simple to do, since common supporters such as Mr. Stone's Project and Castaway search out basic energy.
So the stage 2 itself has a massive strength and mostly inconsequential flaws. What about the pokemon leading up to it? A tall tree has to have strong roots to avoid toppling over in a windstorm.
The stage 1, Gabite, has 80 HP (which is quickly becoming the standard HP for Stage 1 pokemon), 1 retreat cost, and all colorless energy costing attacks. The first attack recovers any energy in the discard, including special energies. This remains one of the only ways to directly place a Double Rainbow energy or a Boost Energy from the discard pile into your hand. His second attack will either place 4 damage counters on any of your opponent's pokemon, or recover 4 damage counters off one of your own. Sure, Gabite is not going to speed your set up in any way. However, a versatile Stage 1 allows for flexibility in your games, and Marvelous Shine is an incredible, yet unreliable, asset.
The only truely weak point is the basic pokemon, Gible. 50 HP is about the lowest acceptable HP for a basic, meaning that a Riolu with a Pluspower will not instantly KO it, nor will a Gabite's Marvelous shine KO Gible. However, Gible fails to present any way to hasten having a Garchomp attacking, and deals merely 10 damage with it's attack. The forced switch is a fun effect, however, the rest of the Gible Family does not directly benefit from such an attack (due to no reliance on spread damage), and the opponent choosing the pokemon to bring active greatly diminishes Push Down's utility purposes.
To conclude:
+Largely safe from being one-shotted due to weakness only being exploitable by late game Blissey and Crystal Shard
+No reliance on more then one energy of any type makes it able to prepare multiple Garchomps or secondary attackers with few problems related to specific energy
+Pure Colorless cost makes support line possibilities wide open
+Stage 1 possesses two remarkable abilities, only weakened by being potentially useless early game
+Able to one shot a vast majority of the pokemon in the game, only pausing at newer stage 2 pokemon
+No other pokemon can match Garchomp in damage output without suffering from recoil effects. Garchomp has no such self-inflicted wounds.
-Stage 2
-Pokemon line does not have any means of searching out the final evolution, nor does it have a means of adding extra energy onto the main attacker (however, Gabite can indirectly do this by retrieving Boost Energy)
-Basic pokemon does almost nothing to support the line
-Despite being easy to counter, the means to turn off Garchomp's most valuable Asset (Rainbow Scale) are found in several decks (Cessation Crystal and Battle Frontier).
Modified: 9/10. You can't run Garchomp without aid to overcome his weak first few turns, and this prevents a 10/10.
Limited: 10/10. Garchomp deals the most damage out of the set for very little cost, can be placed into any deck due to energy costs, and the stage 1's Marvelous Shine lives up to the name in this format. Gible's weakness is shared by most pokemon in this format, thus this pokemon is the single best line to run in Limited Format (MT).
On an unrelated, yet hilarious note: Mirror Matches between Garchomp decks really turn into who get's set up faster, as Garchomp is not capable of one-shotting itself without Lake Boundary being in play (due to there being no basic colorless energy). And no one will play a Lake Boundary in a Garchomp deck due to it not assisting Rainbow Scale and suddenly a Crystal shard on almost any stage 2 is an OHKO for Garchomp. [del]And for some reason, quick reply boxes make it FAR too tempting to just write forever, due to not seeing how long the post is[/del] *SHOT for bad joke*