When you say 'rounded' do you mean coopered or domed? I did a chest with a coopered lid, which I'm sure you could modify for your needs if it's what you are after.
If you mean chamfering the edges with a half-round section, then the quickest way is with a router and bearing-guided roundover cutter.
I would use a sharp block-plane though, finished with sanding. (One of the few times I use abrasive paper.)
I'd also use a plane if I was making a thick top into a 'treasure-chest' type lid. I find the best way is to cooper it and make it curved to begin with, but if you're fussy about finish you need a rounded sole plane or spokeshave to clean up the inside.
If you're on about doing a rounded/domed top then I'm planning something similar myself. Depending on degree of curve will either be bandsaw and plane or straight to plane, spokeshaves very handy here and I think I'll be using the flat bottomed one