I don't have any favourites per-see, but I do have something of a soft spot for the Silurians and I'd like to see more with them down the line. Either in the future after the survivors re-emerge and join with the human civilisation or go off to found off-world colonies of their own. Actually what I'd be really interested in seeing is their civilization at it's peek before they were forced underground.
Did classic Who ever go there? Seems like an obvious thing to do, no?