I always had a soft spot for Grandad - he was totally believable, but that said, Uncle Albert did a supoerb job of stepping into giant shoes as a replacement character.
Uncle Albert has always been my favourite. Some of his situations are really funny... One was when he acted as a cripple for Del's deep penetration back massager, and when it was used on him he started dancing and jumping around (completely breaking from character).
Another was when he was preparing dinner (I believe this was one of the final episodes) and he mixed up the gravy and coffee, leading to much comedy :-).
I vote for Uncle Albert too - I love Fools and Horses and never tire of it, I can watch the episodes over and over again (and do!) and still end up in stiches from laughter. It's guaranteed to bring more than a smile to your face and it will always be a favourite programme.