I don't nearly remember all the pokémon cries I've heard, especially not since I often play the games on muted emulator these days :x But I can agree with Articuno's cry being very beautiful, I remember it specifically even.
There are some later cries I liked too. Hmmm, let's see I think I'll have to watch a youtube video or something to find them.
*does that*
Gen I: I always liked Paras and Parasect, their cries. Interestingly odd. But yeah still. Articuno.
Gen II: honestly I don't particularly favor any cries here haha.
Gen III: Gardevoir's is nice. Sounds a bit mystic. And Milotic, the same feeling there, as well as with Gorebyss. Oh, and Chimecho. Sounds like bells :3
Gen IV: Kricketune, wow. And Lumineon, nice one. Glaceon's is really beautiful, but Frosslass is one step higher. Maybe ice pokémon generally have beautiful cries because ice is thought of as delicate and beautiful?
Gen V: Like with the gen III psychic type cries, I think Gothitelle's cry is mysteriously beautiful. Reshiram's is surprisingly nice and of course Meloetta.
Not sure I can decide on just one favorite, but for the earlier generations, Articuno is certainly a top contestant. But compared to many newer ones, like Glaceon, Kricketune and Finneon, it doesn't quite make the cut.
Still, after this little research on youtube I did, I have to say Frosslass' cry is my favorite, along with some other gen IV ones.