Dragons:
Red- encountered twice as a player.
Blue-as a DM, BBEG in a short campaign.
Green- encountered three times, (4 if you count the templated monstrosity in Shackled City) as a player.
Black- encountered as a player. As a DM, had them in the background, and many Half-Dragon minions/children.
Gold- as a DM. Used to represent a "fire dragon" in a non-D&D game.
Silver- encountered as a player. My best friend (who DMs relatively often) favors them.
Elementals:
Fire- Run as a DM, in the same room as the above Gold Dragon.
Earth- Run as a DM. An Elder Earth Elemental + Greater Abyssal Basilisk = one of my favorite encounters I've ever run.
Golems:
Clay- Run as a DM.
Flesh- Run as a DM.
Hydras:
Hydra (normal), Pyro, Cryo- encountered in the same cave system together in one game. We were not a happy group afterwards.
Robert "Just my Experience" Ranting