I mean, there are so, so many! If we're keeping it Disney, it's going to be Alan Young as Scrooge McDuck, Phil Harris as Baloo, and Bill Farmer as Goofy (though he's not the original). June Foray as Grammy Gummi/Magica De Spell is also a stand-out.
Now, I was an 80's kid though, so without a doubt when I think of the greatest cartoon voice, it's Peter Cullen as Optimus Prime. His, deep, powerful voice made that character the hero of my generation. When they were making hte newer Transformers movie, the producer wanted "big names." Peter went in to read for it anyway, and, yeah, he got the job! Some producer was probably like, "Wow, you do a really good Optimus Prime." "Buddy, I AM Optimus Prime!"