I believe I have the ultimate A500 - not just in hardware, but in software config (hence the name "mr_a500" ;-)).
If you can find an elusive Viper 530 - something I have been searching for for years - then you might have the ultimate. (and I might kill you to steal it :-D - the Viper 530 has 50Mhz 030 and 128Mb RAM!!)
My A500 has:
Viper 520 - 33Mhz 68020, 68882 FPU, 8Mb Fast RAM, 3.0ROMs
Mini Megi - 2Mb Chip RAM
40Gb 5400RPM 2.5" IDE HD
ECS Denise - with Mini Megi gives full ECS chipset
DCTV - for 24bit composite (displaying images, not WB)
Microspeed Trackball - verrry comfortable and no need for mouse space
WB 3.1 - believe me, on an A500 WB 3.9 is NOT the ultimate
I don't consider scandoublers or CD-ROM an improvement so I don't have any. All of my upgrades are internal, so there's no junk hanging off. (except DCTV, but it's just a small black box)