What is the best SSD upgrade for a 27" iMac used for VST plugins?

install Sata Ssd internal , or use external NVME ssd

An internal SATA 6G SSD will have max transfer speed of 500-600 MB/sec—closer to 500 in real life.

An extern NVME in a Thunderbolt 3/4 enclosure can do over 2000 MB/sec.

Easy choice, and you don't have to open a computer Apple did not design to be opened.

Before you invest anything in it, you should keep in mind that macOS 26, due this fall, will be the last version of macOS that will run on intel hardware.

I would not install an internal SSD. Go with an external drive. At least that will still be useful when you're forced to dump your old Mac because it's obsolete.

KiltedTim wrote:

All 27" iMacs are intel based. Intel based Macs will not get any more major updates after macOS 26. It's the end of the line for them. Apple has already made that clear.

You're missing the point.

I did not say that Intel-based Macs would get any major updates after macOS 26. What I pointed out was that the OP's Mac will not be able to run macOS 26. All of the 2020 iMacs had flash storage. None had SATA drive bays. The OP thinks that installing an internal SATA drive is a possibility. While the OP has not yet told us the model year of that Mac, if it has a SATA bay, it cannot be the only 27" iMac that will get macOS 26 – the 2020 one.

It could be anything from a 27" Late 2009 iMac which cannot update past High Sierra, whose internal SATA bay is only a 3.0 Gbps SATA-2 one, and whose USB ports are USB 2.0 ones, to a 27" 2019 iMac which is compatible with Sequoia and which has modern expansion ports. But whatever it is, with respect to macOS upgrades, it's already hit the end of the line.

So my comment was in support of your remark that "Before you invest anything in it, you should keep in mind that macOS 26, due this fall, will be the last version of macOS that will run on Intel hardware." Which in the OP's case, translates to "Before you invest anything in it, you should keep in mind that there are NO future versions of macOS that will run on YOUR Intel hardware. Not even macOS 26."

Reading between the lines, the other posters (and I agree) are telling you that upgrading ANY 27’ iMac is not recommended. Don’t spend the time or money, spend your money upgrading to a new Mac which is Apple Silicon based. What many are doing is bypassing the Apple iMac line all together and getting M4 based Mac Minis which will be FAR more powerful than ANY 27” Intel based iMac.

KiltedTim wrote:

Before you invest anything in it, you should keep in mind that macOS 26, due this fall, will be the last version of macOS that will run on intel hardware.

The OP hasn't told us which 27" iMac he has.

If he actually can "install Sata Ssd internal", that means that it can't be the 2020 one, and therefore, that it won't be getting macOS 26 (Tahoe).

All 27" iMacs are intel based. Intel based Macs will not get any more major updates after macOS 26. It's the end of the line for them. Apple has already made that clear.

+1