The hardest of all destroids is gotta be the Missile Phalanx Destroid in 1/72 followed closely by the 1/72 Monster. Both very expensive, both Wonderfest exclusives. I got the Monster recast from Monkey Nugget which was actually improved from the original. Good luck in your quest, two of everything may be difficult and not necessary, i.e., the monster takes up a whole shelf in the closet, it doesn't have a fancy box either.
So in order of rarity (in my opinion) and approx. value
Phalanx $180-200
Monster $350-$400
Tomahawk $80-95
Spartan $50-70
Defender $40-$50