The Hound of the Baskervilles and The Sign of Four. The latter was the first filmed and was the best of the two despite some tinkering with the original plot. More detailed information on both films can be found in the excellent book Starring Sherlock Holmes.
There would have been more but the producers found themselves in direct competition with Jeremy Brett's Granada series and eventually withdrew.
I do consider Brett's portrayal to have been superior but I also wish that Richardson had done at least one or two more appearances as his Holmes had much to appreciate.
Richardson was doomed to suffer the same curtailment with his next Sherlockian outing as Holmes inspiration Dr Joseph Bell in Murder Rooms.
Wikipedia article for more details.
Richardson seems destined to be most remembered for his turn as Francis Urquhart in The House of Cards and its sequels. This is a shame as he deserves to be remembered for so much more.