Cushing was playing a sort of eccentric scientist in the Dr. Who films - not even an alien which is why they are considered non-cannon. I find the first Dr. Who film way too slow and rather dreadful but the second was somewhat enjoyable. A third film was planned but the second's profits didn't justify it. Cushing played a very similar character in the Amicus film At The Earth's Core.
For great Cushing films I would recommend Hammer's Horror of Dracula, Brides of Dracula, Frankenstein Created Woman, Frankenstein Must be Destroyed, Cash on Demand, and Captain Clegg. Cushing's role in the Amicus film Tales from the Crypt is excellent, and two independent films with great Cushing performances are The Creeping Flesh and Horror Express.
Of course you cannot mention Cushing without Christopher Lee and three of the films I mentioned feature the two horror stars together - they were perfect foils for each other.
On a related comic book related note - Cushing was supposed to star in Hammer's 1975 film adaptation of Vampirella (Barbara Leigh would have played Vampirella) but Hammer was falling apart by that time.