Paul,
A few suggestions for you:
Firstly, another vote for the Ood. Then:
The Family of Blood and Scarecrows – Human Nature/Family of Blood (10th Dr)
Brother Lassar and Krillitanes – School Reunion (10th Dr)
Dalek Sec and Pig Slaves – Daleks of Manhattan/Evolution of the Daleks (10th Dr)
Prisoner Zero, Barney Collins + dog and female patient + 2 daughters – The Eleventh Hour (11th Dr)
Saturnynes (both fish and female form) – Vampires of Venice (11th Dr)
Krynoids – Seeds of Doom (4th Dr)
Reapers – Father’s Day (9th Dr)
The Swarm – Planet of the Dead (10th Dr)
Marshmen – Full Circle (4th Dr)
Mutants – The Mutants (3rd Dr) / Brain of Morbius (4th Dr)
And then of course there could be packs that just included a couple of larger single monsters such as:
Giant Wasp – Unicorn & the Wasp (10th Dr)
The Mara – Kinda/Snakedance (5th Dr)
Anti-Matter Creature – Planet of Evil (4th Dr)
The Fendahl – Image of the Fendahl (4th Dr)
The Foretold – Mummy on the Orient Express (12th Dr)
And that’s just the tip of the iceberg. Here’s hoping you keep producing models for many years to come!