Ultegra 6500 are gone from Shimano range,
https://productinfo.shimano.com/#/sp...tte%20Sprocket
HG50, HG400 and HG200/201 are there. 9 speed cassettes are not going away, you can still get a 5-speed frewheel. (Another question is what Shimano is actually producing given the current problems with supply.)
I think all of them will shift well -- Shimano, SRAM, Microshift, Sunrace. It depends on what matters for you: cassette range and progression, weight, price?
For me the range and the ratio are the most important, and some of the options will disappear as 9 speed is moved down the hierarchy. For example, Ultegra 6500 had a 12-27, it is gone now. I like the progressions without the big jumps in the middle, for 7 speed it was Shimano "M" (13-15-17-19-21-24-28), John Allen explicitly recommends it on
https://www.sheldonbrown.com/k7-7.shtml:
For 9 speed add 11 and 32: 11-13-15-17-19-21-24-28-32, this combination is made by Microshift (H092) (bikewagon has them on new ebay) and Shimano (HG200/201). Also very nice is Microshift H092 11-34: 11-13-15-17-19-21-24-28-34 -- it has 8 cogs in the 11-28 range plus 34t for tougher hills. Now I have HG50 11-30 (11-12-14-16-18-20-23-26-30) on the bike (spotted by ThermionicScott on Universal Cycles).
YMMV, maybe you want something more "road" with less range, like 12-26 or 14-25.