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I don't know; personally I use Cresselia as my main special wall (she covers physical pretty well too, though) and Vaporeon as my physical.
Nobody NEEDS to use Blissey. There are plenty of great alternatives that are more versatile than her.
Well, it's hard to judge without knowing the rest of your team, but Gyarados is much more useful in OU than Scyther is (although I love that fucking mantis myself).
http://www.smogon.com/forums/showthread.php?t=25779
It's fairly accurate and used on shoddy for the most part. But since competitor is, as of yet, unreleased, tiers = not final for the D/P metagame.