I think the most memorable one so far was when I was running a juvenile white dragon against a party of about 6th-7th level.
First they found a frozen horse in a snowdrift.
Then that foggy winters night terrible roars in the fog spooked the horses, and a cold blast hurt a few of the PCs. One of those horses was never recovered.
A little later the party heard that their old village was being terrorised by a young adult white dragon on the cusp of making it to adulthood. Better deal with it quick!
After a couple of interesting and nearly successful attempts to lure the dragon into an outside combat without getting killed, they manage to track it to its lair. Surviving an avalanche started by the dragon, they finally get it backed into its cave. The cave is covered in ice on every surface, so everyone is making balance checks every turn (the priest spent almost the entire melee slipping over in place). At the end of the fight the dragon is severely wounded, but both paladins, the rogue and the cleric are all down, only the sorcerer is left. The dragon lunges forward and bites the sorcerer, trusting to his spell resistance to protect him from those stupid wispy little flames surrounding it. The white dragon takes 2d6+14 damage and dies (double damage from the fire shield), the sorcerer is reduced to disabled (0 hit points) and he crawls from PC to PC, stabilising each of them before they actually died.
Great build up, brilliant final fight and everything came right down to the wire. Thank goodness White dragons are pretty thick
(so far in my campaign the PC's have fought 1 wyrmling white dragon, 2 wyrmling black dragons, 1 young adult white dragon, 1 young blue dragon, 1 young adult green dragon. The party are now average level ~11th and are heading off to tackle another dragon. I wonder what colour it is? I wonder how old it is?

)
Cheers