The Value of Legendary Pokemons in the GTS.

So, with Pokebank open, I decided that I want to complete the National Pokedex. To complete it, you'll have to obtain all the Pokemons from the National Pokedex except for event only Pokemons (like Mew, Celebi, Jirachi, etc.). Obtain means you would have to own it before. If you trade away that Pokemon it is still recorded in your Pokedex as you owning it before. Which is why there is a method people use to trade legendaries called 'Touch-Trading', where one trades to another a legendary Pokemon, and that Pokemon is then traded back to the original owner. The Pokedex still records the data of the legendary and considers that you have owned the legendary (because you did, for a few seconds).

While I do have friends with the complete set of legendary Pokemons, I decide to give myself a challenge by trading away extra copies of legendary Pokemons I have at the GTS, hoping to find good trades to obtain better legendaries. And by the end of the day, I have not only obtained some nice legendaries, but I have also completed the Pokedex entries for non-event legendary Pokemons I do not own.

So, to people having trouble with Legendary Pokemons' data, here's a short guide on how to trade better.

Whether you have access to Pokebank Legendaries or not, the most important thing you should know is the value of your Pokemon. Knowing the value of your Pokemon will enable you to get good trades, and hopefully, reaching your goal of getting legendary Pokemons. One thing you can do is to use Wonder Trade to obtain Pokemons that can only be obtained via Pokebank. If they can be bred, breed it and make your offer on the GTS, and see how lucky or skilled you are at trading.

To get non-event legendaries for trading, you have to know the value of them. So let's rank them from least valuable to most valuable. These are my opinions by the way, so don't take it too seriously.

Zygarde & Mewtwo - Least valuable (Rank D).
They are the most common legendaries you can get. Any player who plays post-game are sure to get one.

Xerneas & Yveltal - Not valuable (Rank C).
Sure, they're version exclusive, but with the release of Pokebank, some may store their legends in there and restart the game, getting another version mascot. Still, they're quite sought after by certain trainers who wants the complete set of legendaries. Also, I think Yveltal is slightly more valuable than Xerneas, as Pokemon X seem to be the more popular version (with Mega Mewtwo X and Mega Charizard X being Fire/Dragon playing a huge role in its popularity).

The Legendary Birds - Not as valuable as you think(Rank C+).
They are quite sought after prior to Pokebank as you'll only get one of the three depending on your starter (and it's quite a pain to get one), but once Pokebank is out, their values drop a lot as the trios are present in many games (Fire Red/Leaf Green, Platinum, Heart Gold/Soul Silver). Nonetheless, some do not know about their value and you can use the birds to get some good trades.

Kyurem, Cobalion, Terrakion, & Virizion - Valuable (Rank B-)
They don't rank as high as other Gen V legendaries as 1. They're not version exclusive, and 2. They're not as hard to get as the other Gen V legends.

Reshiram, Zekrom, Tornadus, & Thundurus - Slightly more valuable (Rank B)
Resh and Zek are mascot legends, and the two Kamis are version exclusive roamers, so they have some value in them compared to those in Rank B-.

Landorus, Cressellia, Heatran, Lake Trio, & Regirock - More Valuable (Rank B+)
Landorus have more value as it is harder to get it in Black/White and can't be obtained in B2/W2 in unless Dream Radar is used. The remaining listed are legendaries that can be obtained easily in B2/W2 and not found in B/W

Regice, Registeel, Regigigias, Latios, Latias - Much more Valuable (Rank B++)
Regice and Registeel's cave can be unlocked by using a key that is exclusive to versions, so you need some with the other game to pass you the key to unlock the cave for the other Regi. Regigigas can only be obtained if you have the Regi trios, and Lati@s is version exclusive in B2/W2.

Lugia, Ho-Oh, Dialga, Palkia & Giratina - Valuable (Rank A)
Can only be obtained in Gen IV, which means more trouble to transfer them over to Gen VI, meaning higher value. Though Dialga and Palkia is slightly more valuable than Giratina since they're version exclusive in D/P even though you can catch both of them in Platinum. Both Lugia and Ho-Oh are equally as valuable as them however.

The Legendary Dogs & the Therian Forms of The Kami Trio- Quite Valuable (Rank A+)
Can be obtained either via Fire Red/Leaf Green or Heart Gold/Soul Silver. Even so, the legendary dog you get depends on the starter you choose, so they are indeed quite rare. The Kami Trio's Therian Forms can only be obtained using the Pokemon Dream Radar 3DS app, and transferred to a copy of Pokemon Black 2 or White 2. Having one of them is pretty rare.

The Weather Trio - Very Valuable (Rank S)
In Gen III, Groudon and Kyogre are slightly more valuable than Rayquaza. In Gen IV's HG/SS, getting Rayquaza is much more difficult than Groudon and Kyogre (which is also quite difficult in it's own right). Therefore they are all very valuable indeed.

Huge Disclaimer:
There are Pokemons (most notably legendaries) that are either cloned or created with methods like Pokegen. Cloned Pokemon do not pose much of a problems other than the having people questioning the Pokemon's authenticity (original or a copy), but a legendary created by Pokegen might to some people. So do be careful when making a trade as you might be trading an authentic Pokemon for a cloned one, or even worse, a hacked one.

So if you have extra legendaries to spare, you can try to trade in the GTS to get the Pokedex entries of Legendary Pokemons. As for me, I don't really mind my legendaries being cloned since I don't use them for battles, it's just obviously hacked Pokemon that I do mind.

Anyways, I shall continue my Pokedex completion quest. So that's all for now.