RE: BEST Legend cards?
Out of the legend cards that are out so far, I'd say Raikou-Entei Legend is the best...although Ho-Oh Legend can splash into quite a few decks (it can even splash into Rain Dance, and can get running immediately with the help of Blastoise UL).
Raikou-Entei is an excellent tech sniper, and should remain fine at least up until the point Claydol is rotated out. For two energy, every other turn (unless something like Energy Pickup is used), it deals 80 damage to all Pokemon in play with a Poke-Power - on both sides. You can nullify this damage to yourself by setting up a Manectric PL, but you can cause a lot of damage for anyone running Claydol, who has exactly 80 HP (or even Uxie with 70 HP). It will also screw up those players who run Mesprit LA and Azelf LA as well, both of which are fairly common. Once someone places a Mesprit/Azelf down, it's hard to get rid of, and basically just sits there (read: easy prize bait). A few SPs have Poke-Powers, but of the ones that do, they usually have 90 or more HP (although Bronzong G can be KO'd or rendered useless by Thunder Fall), but it can still inflict a lot of harm.
Is it viable to use in today's format? Gyarados and Donphan say hi and no. Both of those guys can one-hit Raikou-Entei, so for those seeking to use legends, you need to pursue some other options...