Best ever cliffhanger?

Honestly

Army of Ghosts

(with the Cybermen revealed and the Sphere hadn't opened I just knew and I think I actually said "Heck yes its on now"

Honourable mentions every cliff-hanger in Genesis of the Daleks

Part 3 of Caves

Bad Wolf

Regards

mark687

Dragonfire.

I'll get my coat.

Right, sticking mine to New Who, since whilst I've seen Classic serials, I haven't seen many more than once so it's hard to remember a good cliffhanger from the Classic Era (though, the Dalek coming out of the water in episode 1 of DIOE is the one that sticks out to me).

Bad Wolf - the revelation that there's a huge fleet of Daleks still out there, and Rose is trapped in the middle of them and Nine promises to come rescue her.
Daleks In Manhattan - The Human-Dalek reveal was pretty cool to me at the time, since I hadn't long been into the show at the time. Looking back it's... not great, especially since the second part was disappointing.
Utopia - I AM THE MASTER. That's all.
The Sound of Drums - The Master taking over the Valiant, capturing Martha's family, the Paradox Machine, the old Doctor and the Toclafane destroying the world as Martha watches on... Yeah, that was pretty exciting at the time. Of course, it's another one where the resolution was disappointing.
The Stolen Earth - Ten being shot and beginning to regenerate was... yeah. Wow. I vaguely remember seeing photos of them shooting the scene, but I wasn't expecting him to actually regenerate.
The Pandorica Opens - Yeah, the photos of the Alliance in the Pandorica Chamber had already been leaked, but seeing them all together on screen was just insane. Put that together with Rory shooting Amy, River being stuck in an exploding TARDIS in a time loop, and the Doctor being locked in the Pandorica and being blamed for the end of the universe. Yeah, that's probably my all-time favourite Doctor Who cliffhanger.
The Almost People - When Amy wakes up in that pod... thing and it becomes obvious that the Amy that's been travelling around with Eleven and Rory isn't the proper Amy... and the fact that the real Amy is pregnant and ready to pop. Pretty shocking.
The Name of the Doctor - The classic Doctors showing up was pretty thrilling enough but John Hurt being revealed as "The Doctor" at the end was the icing on the cake and was a great way to lead into the 50th.
Dark Water - It was obvious enough, but the revelation of the Cybermen being in the tanks and Missy being the Master was still really exciting.
World Enough and Time - I'd figured out that Bill was probably going to become a Cyberman, but it was still pretty horrifying seeing her reaching out for the Doctor. And the Razor reveal was still a little surprising for me (I thought it could be the Master in disguise, but I'd decided it was probably someone working for the Master instead) so it was fun, and seeing him making an alliance with Missy was great too.

Bad Wolf. The Daleks had been set up so well earlier in the season that the tension pays off brilliantly.

Caves of Androzani Part 3-'So you see, I'm not going to let you stop me now!' Chills every time.

Sarah Jane falling from the scaffolding in Genesis of the Daleks, even if the resolution was a bit of a cheat. That was the one that really imprinted itself on me more than any other, back in the day. The Fourth Doctor being held underwater in Deadly Assassin would probably be runner up for me, even more so than the Doctor about to shoot the president cliffhanger in part 1, and for much the same reason.

As far as New Series goes, John Hurt popping up at the end of Name of the Doctor, because I had managed to stay completely unspoiled beforehand, so never saw it coming. So that one blew my mind a little bit, not just a new Doctor, but John freakin' Hurt.

Those would be the top ones for me that I experienced 'live' at the time. But if it were opened up to others that I just really like, but saw well after the fact, then the TARDIS blowing apart in the Mind Robber has got to be right up there for me as well.

I'll go for a classic: the end of "Part Three" for The Caves of Androzani.
Irrespective of incarnation or period, that little exchange there has become what I think every writer strives to do with the Doctor. He knows he's going to die, he knows there's no guarantee of success, but he's going to try anyway because all that matters is that one life. To hell with Stotz, his gunmen, Jek and the two Androzani worlds, Peri is getting out alive and that's all that matters.

In no order:

Caves 3: others have explained it.
Remembrance 1: seriously, who wasn't freaked out by that?
Utopia: the Master, stolen TARDIS, insane cannibals, end of the the universe....
Bad Wolf: the Doctor freaks out the Daleks by having no plan at all then, "Rose... I'm coming to get you."
The Stolen Earth: did anyone else think that we'd been trolled with a regeneration?
Warrior's Gate 3: clever use of time travel puts the Doctor in even more peril.
Seeds Of Doom 3: Sarah is being used as a sample for the second Krynoid seed.
World Enough And Time: Cybermen, The Master AND Bill???
Ark In Space 2: I can trace my love/hate affair with body horror to this moment.

Caves of Androzani Episode 3, as everyone has already said.
In addition to that, the Ark in Space Ep. 2, in which Noah takes his hand out of his pocket and discovers the larva, is firmly embedded in my memory.


Off the top of my head, other highlights include The War Games Ep. 1, Mindwarp Ep. 4, and Terror of the Vervoids Ep. 2.