Yes, the "coolest" is a matter of opinion.
I'm thinking of tank #3 and am browsing the web for ideas and themes. If you've seen my all artificial "planted" tank or freshwater coral tank, you know I like stuff that's different.
What are the neatest aquarium safe decorations or ornaments you've seen?
These are some of mine:
Really neat, but the price, OUCH. I could make this look so sweet with an Asian theme and lots of thick stemmed plants/wood.
Mountain Drawbridge Ornament hose 20" L - Rock Cave | eBay
Another great mountain scene from the same place:
Mountain View Aquarium Ornament tree house Cave Bridge | eBay
I've always been a big fan of the sunken galleon when done right:
OSI Galleon 2pcs Sunken ShipWreck Aquarium Decorations Ornaments | eBay
Reenact a scene from Lord of the Rings!
Show me what themes and decor you find attractive.