Pertwee-era Judoon! ;)
Okay, one cheap gag about old special effects is probably plenty. I'll be good.
One thing I'll note - of the eight monsters I used, the Doctor knew about four of them before their first screen appearance. Possibly five considering he might have known about the Weeping Angels beforehand... So yes, "the Doctor is unsurprised by a new monster" seems a bit common these days. So some of these could be first meetings.
Worlds At War (1996)
The Eighth Doctor and Lucie find themselves on the run when his attempts to broker a peace between the Seven Nightmares system and the Molasar invasion fleet are sabotaged and the ambassadors murdered. Chased by the Judoon, intergalactic police for hire, can the travellers clear their names and stop an interplanetary war?
Bit of a random one, but hey, it's Eight, almost anything can be new to him. I don't believe Big Finish's licence would let them actually bring in 2005+ monsters to the earlier Doctors' audio adventures, but y'know, more Judoon, always fun.
Heart Of Stone (1989)
The Seventh Doctor and Ace confront the Weeping Angels, beings from the dark at the edge of Time. In an abandoned house on a wintry coastline, they have to fight their way between three different eras of the house's history or be trapped forever as the eerily-still Angels take the TARDIS and conquer time as we know it!
The Weeping Angels would fit into the Seventh Doctor's "serious" seasons rather well - I can almost see this one as I think about it. Lots of isolated old houses, monsters that were creepy rather than colourful, and threats to reality itself being included as ways of upping the stakes.
The Sixth Doctor and Peri are shocked to find that an entire planet has been stolen by the Slitheen, an intergalactic crime family. They have to sneak onto a crime planet to disrupt the auction, attended by the worst scum of the universe, and save the stolen world!
Big, brash, colourful and prone to rather broad jokes, the Slitheen wouldn't look out of place in a Sixth Doctor adventure. Throw in some gags at the expense of yuppies and it could be a minor classic of the era. Death by great big 80s mobile phone, perhaps?
The Fifth Doctor and Nyssa are drawn into conflict with the Sycorax, savages and thieves of the spaceways, as they threaten the colonisation of America with their alien witchcraft!
As with Ten, so with Five. A bit of swordfighting and a thrown
The Face Of The Dark (1976)
The Fourth Doctor and Sarah Jane find themselves trapped in a space station orbiting the Sun, but where it constantly seems dark, and the crew are being murdered, one by one, leaving only their skeletons picked clean. The travellers realise that the Vashta Nerada are aboard, and plan to kill the sun and feast upon the Earth below!
Vashta Nerada might be tricky without CGI, but there are lots of practical lighting effects that can create anomalous shadows. And a faceless invader picking off a small isolated cast is a Who classic so they could appear then as easily as now.
Secret Of The Daleks (1971)
The Third Doctor and Jo discover a small advance force of Daleks on Earth - and are surprised to find that they are not alone in fighting them, as a pair of seemingly young but white-haired people sabotage the Daleks' plans with an explosive that could destroy London as well as the intended targets!
The Movellans are a nice simple idea - an enemy of the Daleks is not necessarily a friend.
Terror From The Deep (1968)
The Second Doctor, Jamie and Zoe arrive in an isolated fishing village to find that its people live in mortal fear of the Devil lurking beneath the waves...
Yeah, only going back one Doctor, but an overblown Hammer style title seemed the way to go.
The Master (1966)
The Doctor, Ben and Polly face the Master, tyrannical ruler of a distant planet, who claims to be from the Doctor's home world...
The production team added the Master during the Third Doctor's exile to Earth to give him a recurring villain to bounce off, but considering how they feel about each other, a retroactive first meeting would be a lot of fun...