IMHO, it may be an O’Brien D.
I was struggling with my Clarke before I got the O’Brien. It is MUCH easier to play. Better balance between octaves, less air required, somewhat softer volume overall, heavier – it doesn’t try to jump out of my hands, and the curved airway lessens moisture problems. The holes are much easier to find on the solid copper than they are on the thin tin.
More expensive than the cheapies, but well worth the money in terms of playability for a novice. Well, this novice anyway.
Disclaimer: I haven’t played a Feadog, Generation, Walton, Sweetone or any of the other low-end whistles.
Glad you like the O’Brien. It’s hard to say what the best beginning whistle is. I’d say it’s any whistle that doesn’t require difficult breath-requirements, especially for tunning. Hey, I started on a Chieftain! 
If you like the O’Brien, you’ll likely enjoy one of Jerry Freeman’s Mellow Dogs. It only looks like a cheapie: don’t be fooled.
The O’Brien’s great, giving you a chance to muck about in different keys without breaking the bank. Get an Eb tube if you want a really responsive tooter.
Did you mean to ask what’s a good whistle for a perfect beginner or what’s the perfect whistle for a beginner?
If a perfect whistle exists, it’s stuck in a rock somewhere in England waiting for some whistle-squire to pull it forth.
But, there lots of good whistles for beginners. My first whistle was a non-tuneable Dixon in ABS/polymer, key of D. Sounds very warm and nice, very easy to play, makes a beginner sound better than they should, durable, not too expensive.
Good luck in your search of the English countryside.
The Dixon D one-piece is a very good beginner whistle. Sweetones also are easy to control, so possibly suitable for timid beginners…
…but a Feadog would be my recommendation for a serious beginner. Unlike the other two it needs a degree of breath control to shape the tone and keep the squawks at bay. Also, it is loud enough for a session, nicely in tune, and can be adequate for any level of player.
I guess the same is true of Gens, but personally I don’t really like 'em, except the older Eb ones.
Personally, I think the Mellow Dog is more for the advanced beginner than the absolute beginner. It took me a while to learn to handle mine. Once I got the knack though it was great. I played it in last night’s session, actually! 
I’d say it depends on how much you are willing to invest as a beginner. Of the whistles I’ve played my Copeland D is by far the easiest. The indentations surrounding the holes provide nice little easy-to-find landing sites for my fingers, so I make far fewer mistakes on it. I don’t find the breath requirements difficult. The only difficulty with breath comes when switching to another whistle and adapting to its requirements. And because it sounds so much better than the cheap whistle I tried at first, I am more inspired to play and improve.
Marcia
Clark SweeTone… cheap, well tuned and easy to play. For $6, how can you go wrong? Unless you have a wad of money… then it’s another matter completely, heh heh heh…
DAve
I’m happy with my Clare D whistle. 