What is everyone's favorite classic Amiga expansion hardware?
I'll start:
Amiga 1200D:
Blizzard 1230-IV with SCSI-IV kit.
It is the best upgrade one could get for a non-towerised original desktop unit, as it provides you with a decent accelerator, DMA SCSI, and ram upgrade, and it runs cooler than the bigger brothers, when used in desktop case (B1240/B1260). It provides a nice speed-up compared to stock units, and since you will be stuck with AGA, I don't think one should go with B1240/B1260 in this case as it will provide little improvement over 030 (speed wise) when using AGA and OS3.x, and they definitely generate more heat. OK, there will be some noticeable improvement of course, but the real power of 040 and 060 comes to light when using them with a graphics card. That's when you notice some significant difference. ;-)
Amiga 3000/4000:
Cyberstorm MK2 and above.
You have Zorro slots. You can install a graphics card, so why not complement your big box Amiga with a nice 040/060 accelerator. I would say that Cyberstorm MK2 would be a good place to start, as it is very reliable and much better than its predecessor MK1, and if you want SCSI you can attach it, and of course the later models MK3 and PPC only add more spice to an already established well made design.
The second pick for A3000/A4000 would be various PCI busboards (they are all very good, IMO), but the crown of course goes to the Mediator, as far as elegance and software support go.
Well, I could name plenty of more stuff, but I will leave that to other posters. :-)