I would then considering doing a time-to-exhaustion test where you ride at your current or what you think your FTP is for as long as possible. You want to know what your FTP is, go and ride at it as long as you can and you’ll find out. There’s no playing the “I’m VO2max challenged” game trying to sort out which test is best for you.
The thing with the ramp tests and shorter 8 and 20 min versions is that they are estimating your FTP based on a percentage of normalized power or highest 1-minute power you can achieve–they are not truly 100% accurate. I’m not saying a TTE (time to exhaustion) test is truly accurate either, you could have an off-day where you underperform, but what could be more accurate than trying to ride at your FTP for 45-60 min?
Some people throw out the excuse that they are stressful (not really, they are around 100TSS depending on how long you go) and take a lot of mental fortitude. However, this is the nature of training–it’s not meant to be mentally easy. Whether or not one test is more mentally challenging than the other is totally personal. I do like the idea of having a very reliable FTP figure though and for me the TTE test is well worth the 75 min investment.
I would take a look at this thread and decide for yourself if you want to try it. I did it yesterday and was pleasantly surprised at how mellow it was. The last 10 minutes were a struggle, but it overall it was a lot more friendly than the 8-minute tests I’ve taken. And it’s a great workout to boot.