Change is flying (tossing in my 2cents).
As many others I would vote for the turboboard to be the greatest uppgrade to an Amiga. There are alot of different boards out there and of cause CSPPC060 and BPPC060e+ hit the highest next to the different 060 solusions but even a 030/50 card with SCSI is a good uppgrade. 040 don't hit high enough due to the difficulties it's heatproblem brings along.
After turboboards the different PCI solusions out there are the most significant uppgrade to the classic Amiga allowing for newer, easier to get and faster gfx cards, netcards, soundcards and tvcards to name a few. Zorro is just outdated and full set of hardware for Zorro usually runs higher than the cost of a PCI solusion fully set (which is also better).
Third I would like to add harddrive expansion for the early classic models. When it came it was so hot one didn't dare of touching it... never less pay for it but hay it was hot! Mmmm... Thinking back to the times I drewled over my friends sweet sweet GVP530... ahhh...
