This is a fairly easy topic. My favorite dragon type is probably the black dragon. They've got really interesting lairs, they breathe fire rather than acid, and they're more likely to be swimming than flying.
Black dragons typically lair in swamps. The default dragon lair is, at least in my experience, some kind of mountain cave, maybe with some lava or at least high temperatures (because the default dragon breathes fire), and is kind of a standard dungeon as far as environment goes. Black dragons throw that out the window: underwater entrances and emergency exits, swamp-dwelling monsters like lizardfolk, and utter darkness (rather than having fire burning like in a red dragon's lair). I love the swamp theme: air so humid that the walls are dripping, deep pools of water that may contain monsters or hidden treasures, moss and pond scum growing everywhere, and complete pitch blackness.
Going along with the swamp theme, black dragons always have lots of water in their lairs. Their hoard might even be deep in a pool. The dragon could have a much higher chance to surprise the PCs; it's tough to miss a massive dragon sitting on a pile of gold, but if the dragon only has its eyes and nostrils above water (like a crocodile), it's much harder to spot.
On top of that, black dragons spit acid instead of breathing fire. Their breath doesn't light up the darkness – it just eats through the PCs while they fumble around in the darkness. I'd even rule that the acid could put out a torch if it hits a PC carrying one.
Black dragons, to me, seem way more like sneaky, devious opponents than standard red dragons; where reds may just rush into combat, hoping to burn and claw their enemies, my black dragons use darkness and water to get the drop on adventurers, taking their time to attack rather than blatantly exposing themselves to attacks.
As an aside, my fondness for black dragons might stem from my first experience reading about dragons of any kind. I read Dragons of Autumn Twilight by Margaret Weis and Tracy Hickman when I was ten or so, and the scene where Onyx rises out of the well to attack the group, casting a darkness spell and breathing acid on Riverwind is one of my favorite fantasy scenes ever.