Hi!
For JPM it really depends.
Sometimes the cultivar are the same, so you should have a tree with the same characteristic over and over but it depends!
By asking the ''best for bonsai'' what does it means for you? which one has the smallest leaves or will be the easiest to maintain in proper conditions where you live?
The more I learn about maples and the more I think about the conditions I can provide for the tree especially during winter, on the final size of the tree I want (Thu the importance of a smaller or bigger basic leaf size) and the color during a given season.
Leaf size is ''adaptable'' yes to a certain point but even among cultivar of Acer Palmatum you can end-up with a slightly different size of leaves ''naturally'' depending on a lot of factor.
As for example
@Lorax7 is suggesting Katsura, for me my Katsura is the tree that tend to produce the biggest leaves of all tree in my small pre-bonsai collection. Whatever I do it always have a tendency to produce bigger leaves.
If you order online you will have to be flexible on the leaf size of the tree you will receive if you can choose a tree in particular ask for pictures of everything!
Whatever your choice the thing I try is to always gets cuttings and as much info's on the parent trees, no graft (I made the mistake once..)
There is obvious bonsai appropriate cultivars amongs Acer Palmatum specie but judging in terms of beauty and depending of the season! my favorite one's are (Personal taste):
-Sango-kaku (Always beautiful every seasons, easy to grow)
-Deshojo (Beautiful leaf color in Spring/Summer if you take time to look at it)
-Bloodgood / Emperor (Leaf's are impressive in autumn you can't miss it)
-Katsura (For summer color)
A Bloodgood emperor in term of leaves color. This tree is not my favorite one during summer / the tree is grafted....
I didn't really know what I was doing but the owner was about to throw it in the trash! So I saved it! its color is beautiful and steal the show at the end of Autumn.
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If I go by the leaf's size this little guy wins along with my Yuki Hime.
Its not a beautiful tree, color are not exceptional, green in summer yellowish to a flat red in Autumn, A simple Acer Palmatum but that has a unique characteristic it has naturally really small leaves! The biggest are about a quarter to the smallest about 10 cents sized leaves.
I did not prune the tree to attain this leaf size.
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