I think the best Unova shiny is Zorua. To me the blue in his shiny looks mystical like a tribal ocher a shaman in a cave painted his face with. Not only is this my favorite gen 5 shiny but Zorua is one of my favorite shinies ever. He'd be in my top 5 with Shiny Milotic, shiny Rayquaza (regular or primal) , shiny Rapidash and shiny Mega Gardy.
Back to strictly Unova i was pleasantly surprised by Gigalith's shiny. He's one of my favorite gen 5 Pokemon and I enjoyed seeing him turn into living treasure, its like turquoise being mined out of his back. I think the Haxorus line is also interesting, can't decide whether I prefer shiny Haxorus or Shiny Fraxure-- when in doubt add black to a dragon and all will be well :)
Last but not least let's talk about shiny Scolipede. He looks even more poisonous with those neon markings. I just feel like I've taken a trip to the rainforests and have stumbled upon a tree covered with poison dart frogs and other exotic creatures when I see this shiny.