For me, it would be acer palmatum 'Summer Gold'. Probably my fav yellow because of the way it holds up in not only lots of sun...but extreme muggy humid heat.

~AL

What size tree are you looking for? 'Shidava Gold' and 'Lima Gold' are interesting smaller trees. Moving up in size a bit, there's a palmatum called 'Aureum' (distinct from the Shirasawa clone). The yellow color of these plants does fade to light green as the leaves age and for gold color all season, maybe you'd be best with 'Summer Gold' as suggested above; I haven't seen it in person and so can't really recommend it.

Alex

Al I would love an update on your Summer Gold. Your post in the Spring instigated an inquiry from me to see how it does in full sun. kb_guess offered me a new graft from that thread. Thanks again Keith. I have to say I just got it a couple weeks ago and am astounded that a youngster graft was so full, but one thing I also see is some red in the leaves. I can't speak about how well it's going to do yet, as it's just a baby and only in the ground a couple weeks which is why I'd love to see an update on your old post with a photo of yours now and how it's done this summer when you have time.
Cher

Acer p. 'Bihou' might be a great addition to your yellow collection. Yellow in the middle of winter! Though it will have green leaves in summer.
Also check out this article about winter bark color

Here is a link that might be useful: Japanese Maples in Winter

Hey Chohio.....Sorry i forgot,but i will post a photo of my 'Summer Gold' this spring!