The Founder of the Feast, of course. Obviously, he's been softened substantially from Barks--if he behaves badly in a Ducktales episode, you know he'll repent by the end, and you never see episodes concluding with him threatening to inflict bodily harm on family members. In spite of that, though, I don't think it's a bad portrayal, and Alan Young's voice acting is fine. The lack of emotional complexity compared to the original version is notable, though.
The one thing I can't quite get over is the fact that he has a mansion filled with valuable bric-a-brac (including bronze statues of himself), a limousine, and a chauffeur/butler. That is just wildly out of character, by comic or cartoon metric. I think it's more or less accepted in comic-world that he lives in an annex of the money bin, but if not, you would expect much more spartan digs. I think three factors contributed to this: first, the people who conceived the show simply didn't have particularly good grasp of the character, plain and simple. Second, they wanted to convey "great wealth," and they probably thought it would've been confusing if he didn't have the most conspicuous signifiers thereof. And finally, there's the matter of wish fulfillment: HDL are there for kids to identify with, and it's much more fun to live in a mansion than some hovel, even if Scrooge is tight with the spending money. So there you go.
Huey, Dewey, and Louie Duck
I read that the voice actress, Russi Taylor, also does Martin and a few other characters on The Simpsons, and there, of course, she sounds nothing like HDL do--so kudos to that. Impressive range. Fact is, HDL probably would sound like that, even if I'm a little uncomfortable about giving the comic characters voice actors at all.
The portrayal of HDL is kinda all over the map. Sometimes they're smart Junior Woodchucks (though the occasional "puzzles" they have to solve are always kind of on the inane side); sometimes they're just regular, rambunctious kids; sometimes they're amazingly dumb; and sometimes they're just plain dicks. Of course, there was variation in Barks' portrayal of them too, but nothing quite so schizophrenic. Still, when they're good, they're pretty good. Ducktales does a decent job with the basics.
It's not really enough to say that Donald was eliminated as a regular character just because he would've been a third wheel, given that Launchpad frequently stands in for him. Clearly, another big factor is his voice: okay for shorts; might grate a bit after prolonged exposure in a twenty-two minute show. Still, fuckin' eh, man. There is a part of me that says: if you're trying to adapt Barks, you can't downplay Donald, dammit. I already wrote a bit about how I think his absence sometimes has an unbalancing effect. And on those infrequent occasions when he is present, he generally makes a decent impression, even if, by definition, his portrayal isn't wholly Barksian. I would probably, as with Scrooge, complain about lack of emotional complexity, but his appearances are infrequent enough that it's sort of a moot point. I would've liked more of him, is all I'm saying. I think if they'd made more episodes, it wouldn't have been a bad idea to have him get out of the navy and resume residence in Duckburg with HDL. Seems to me that that would've opened up some good plot ideas.
…Who only has a substantial role in one episode, of course. Not saying his portrayal is bad, per se, but given that the entire purpose of Gladstone is to be obnoxious, the usual Ducktales softening effect seems counterproductive here. You can see why they didn't use him more often: if he's not serving as an antagonist to Donald, what is he doing, exactly? You have to admit, though, it would've been super-cool had they brought him and Donald together in an episode for a knock-down drag-out fight. Failing that, Fenton probably would've also made a good adversary.