I think there are two slightly different types of gage - green gages, and yellow gages. The green gages seem to be smaller and rounder. The yellow gages such as Oullins and Coe's Golden Drop seem to be slightly larger. Jefferson is another yellow gage worth trying (possibly descended from Coe'). It's probably debateable whether the yellow gages are gages or plums, but they are definitely European rather than Japanese and in my biased opinion have a far better flavor!
I cannot claim to know "The Best" but last summer there was a tree at the HOS Arboretum bearing a variety called Beauty that was very favorful and with considerable flavor complexity.
Other individuals also shared that opinion -- the tree was on the fence line and people snitching plums off the tree broke a fairly subtantial branch to get their fill. I salvaged a bunch to take home and eat on the drive home, but made the mistake of leaving them on the front seat of my pick-up truck while I went into get a newspaper. When I returned, my Airedale had eaten them all.
So as a review, some fruit thieves, my dog and me all consider Beauty to be a good choice -- red purple skin with intense yellow flesh approaching 2" in diameter (maybe just a bit smaller). A smallish sized pit that in not-freestone.
LeeN
PS One of the very real benefits of doing volunteer work at the HOS Arboretum is you get to taste lots and lots of varieties.
Pat …keep in mind that whatever plum you go for will need a pollinator…. So a ‘named variety’ will be very helpful for finding an appropriate match. I’ve got …twenty+ year old “Ozark Premier’s,†likely my favorite Asian plum, with no viable pollinator - thus little to no fruit… What’s necessary is the right ‘variety’ (Asian or European) blooming at the same time - and lacking close genetic ties, such as sharing a parent.
Heck, it’s not easy when you know their name! …Lots of inbred hybrids out there with few viable or reliable pollinators… And pollination ‘charts’ have been very unhelpful to me … to the point of purchasing a supposedly cross-pollinating variety, raising it to a tree – then finding it doesn’t do the trick (they’re close relatives!). Result – two worthless trees …and still trying 
Lee: “One of the very real benefits of doing volunteer work at the HOS Arboretum is you get to taste lots and lots of varieties.†I’d lobbied as hard as a young member could to get that located on the West side… glad it’s working out for some of us, and what a fantastic resource to Portland proper…
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