3 years ago. (And one of those appearances, if not both actually, was just out of them wanting to save money on a new costume/alien.) Their ship has sailed, their time is over, I think.

Three years ago isn't that long. I don't think the Ood will return yet but I'll be surprised if they don't once a new showrunner takes over.

Funny that. Cos, say for example the Daleks were off-screen for 3 years, you'd be saying the total opposite - that 3 years is too long. Just sayin'. But I think we have to wait quite a while anyway before we see what monsters and such are truly remembered and considered 'classic', etc.

Daleks are the main reason for the show's success. The show wouldn't be here if it wasn't for the Daleks.

I don't want to derail the thread. And I'm not disagreeing there. But classic Who managed without a Dalek story every single season. Jus' sayin'. Maybe that helped them become classic and iconic, cos Dalek stories were therefore more like events (after the 60s anyhoo).

Great topic!

There's quite a few in my view:

The Reapers. Creepy and effective.
The Ood. Very multifaceted, great look, can work equally well as angels or villains. Spooky voices, too...
The Hoix - would love to see them return to DW after their too-brief appearance in Love & Monsters. Genuinely unsettling.
The Judoon. Love 'em. The multi-Rhino equivalents of Gene Hunt.
The Weeping Angels - in Blink, anyway.
The Silence - horrid. I approve.
The Whispermen - as above.
The Clockwork Droids - in both Girl in the Fireplace and Deep Breath. They work really well in my view. Creepy, dead-eyed and beautiful.
Thing Under the Blanket in Listen. Oh my God...
Skovox Blitzer. Looks rather cute, like a tin owl perching on a supermarket trolley. Be under no illusions though- it can destroy the universe!
The Boneless. Very disturbing villains. Very.

-Weeping Angels definitely
-Silence definitely

-Whisper Men have a great potential but they appeared in only one episode so I wouldn't give them a classic status yet, but if they begin appear more frequently and with good development, they could definitely become a classic monsters.

Totally disagree. The Whispermen were just generic henchmen. I don't see what more could be done with them; the Yeti could easily have taken their place in Name Of The Doctor.