There are a fairly large number of monsters that are both iconic and favorites of mine, yet I've never actually used them in an encounter as a DM.
Often this owes to their power; one doesn't pull a Demogorgon figure out of the box every session, after all, and there's a good reason most players have more experience with kobolds than storm giants. I've also run very few campaigns in which characters got above 8th level (especially in the old days when leveling up was a much more arduous affair), so usually facing powerful monsters simply isn't something the PCs can do.
Still, there's a few critters I've always wanted to use but haven't: beholders, catoblepases, umber hulks, su-monsters, to name a few. Other creatures, such as the purple worm, saw use in my 1st Edition games but not afterward in latter editions. The arrival of Second Edition in particular had a big impact, because giants and dragons got a huge boost (and have been powerful foes ever since).
This past weekend, beholders (yeah, two of them) entered one of my campaigns for the first time. No, the PCs were still low-level. It was more of a foreshadowing encounter, a little something to make the players think ahead and to alert them to the challenges ahead. Little fish in a big pond and all. After a little thought, I decided I wasn't going to be shackled every single encounter by that 3e and 4e "CR" mentality. My current group, as mentioned in my last campaign post, is a very sharp group and they're smart enough to know when to hold 'em and when to fold 'em. And in this case, they kept their heads down.
I can't speak for my players, but for me it was fun to finally plunk the figures down on the mat and play these far-from-the-norm uber-baddies. After a while, one gets tired of kobolds and such!
If you're a DM, I suggest you find that never-used favorite, be they high level or low, and find a way to work them into your game. You might just enjoy it.
Now where did I put that purple worm figure of mine?