I'd hesitate to say there is a "best" whisky, but there are whiskies he will like and ones he does not. Some people swear by Lagavulin and Laphroig which are very, very peaty/smoky (they taste like TCP to me :sick: ). They are way too much for me, Talisker has the maximum amount of peatiness which I find palatable.
The standard Glenfiddich is, dare I say it, quite bland (to me, at least) but the 15 and 18 are much improved. But, if your dad likes the sort of flavours that you get in Glenfiddich, then he may prefer the lighter whiskies, such as Dalwhinnie, The Singleton or Oban. Dalwhinnie is my personal favourite, as it's delicate and sweet - you can appreciate the taste without feeling that your tongue is being assaulted with a lead pipe, but again, it's all personal preference.
I'm sure someone will now accuse me of being a heathen with no taste, but in my view, you just buy what you like and damn the snobs :grin:
By the way, if you have more than a passing interest in Whisky, then
101 whiskies to try before you die, By Ian Buxton is worth a read