the Vivitar 28-105 (the one on the left).
I've acquired several interesting lenses and had a few pleasant surprises, but overall if I'm keeping just one it's the Kobori (serial#77xxxx) A-enabled lens. Some were dismissed after a few tests, but the top three were worked over more thoroughly.
Here's a basic list, more or less in order of finish (#s 2-4 were very close):
Vivitar 28-105 f/3.5-4.5 (Kobori, serial number begins 77)
Rikenon 35-135 f/3.5-4.5
Sakar 28-200 f/4-5.6
Vivitar 28-85 f/3.5-4.5 (Kobori, no 'A' setting)
Promaster 28-80 f/3.5-4.5
Rikenon 28-100 f/4
So why is the 28-105 on top?
The A option is a big plus, although the Sakar matches that and has better range. On the other hand, I like the sub-500g weight of the Vivitar; 800 grams is just too much bulk for me to carry cheerfully! It also has the curious minimum focus setting which varies by focal length: the more you zoom the closer the focus, ranging from just over 8' at 28mm to 6' at 200. I had a newer Tamron 28-200 in my Sony days that was weak beyond 120mm, so I was quite surprised to see one of the earliest versions of this type perform so well!
the other lenses are all manual-aperture only..
The 28-105 goes close and reaches 1:5, but they nearly all do and the Ricoh goes 1:4. I feel the 28 wide end beats the Rikenon 135 tele, and colors with the Ricoh lens are biased warmer even using auto WB. How that happens I still don't understand, but there it is. The Sakar does the least with close focus, but given it has the longest FL range it had to compromise somewhere.
Color fringing is nearly absent on the 28-105, also true on the Sakar but a bit more CA on the 28-85 and Ricoh.
The one-touch zoom/focus design won me over, since part of my manual-focus motivation is for video use. I'm well covered in AF now, but video doesn't care and using one hand feels right with video shots. So despite their other good points this removed the Promaster 28-70 (despite the A setting) and Rikenon 28-100 (another heavy one) from more rigorous tests.
Last and probably not least: a Kiron 28-105. These are very well thought of, and it should have rated near the top. Sadly, it came paired with the 28-100 and is a Canon mount - so it's not for me. The images on the auction were vague, I didn't know until after I had won which lens was the Canon.. oh well, I'm glad I got to sample the 28-200.
Beautiful.. but which lens? Even I'm not positive, but I believe it's the 28-105.
Having these lenses to square off against each other has been interesting, and while I could still do better I feel I got a very good lens, and have some degree of proof. We shall see how much the market allows me to recapture on these; a few were real bargains so I'll just about break even I suspect.