I do not like the term GOAT because it is always be controversial. We can find a good bunch of guys and girls whom I could assume are amongst:
Dave Scott, Mark Allen, Frodeno, Gomez and Alistair Brownlee. For me they are the GOAT till now and Blummenfelt is not that far
Scott: 6x IM worlds (but no ITU success because it did not exist)
Allen: 6 xIM worlds and 1 ITU worlds (no olympic medals because triathlon was not olympic then)
Frodeno: 1x olympic gold, 3x IM worlds, 2x 70.3 worlds
Gomez: 5x ITU worlds, 2x70.3 worlds, 1xolympic silver
Brownlee: 2 xITU worlds, 2x olympic golds
Blummenfelt: 1xolympic gold, 1x ITU worlds, 1x IM worlds, 1x 70.3 worlds
A fast analysis
Best IM triathlete: Scott or Allen
Best ITU triathlete: Gomez
Best triathlete in short distance at his best: Brownlee
Most complete: Probably Blummenfelt
But still people thinks Frodeno is the GOAT (thinking mostly that because of his success in IM)
we cannot compare apples to oranges. Back in the 80s there were not ITU worlds, no 70.3 worlds, no olympics. In the 90s there were not 70.3 worlds neither olympics (80s and 90s with no drafting). We also know the competition back then was smaller than now. I am spanish and when people talk about the GOAT in soccer do not think about one but 4 or 5 (Di Stefano, Pelé, Cruyff, Maradona and now Messi).
What I have learnt in 20 years watching triathlon (mainly ITU till 6-7 years ago) and old videos is that there is no GOAT, but the most dominant guy what I have seen in a race is Alistair Brownlee. Then I would say that Allen is for sure the legend, Gomez the most consistent guy I have ever seen, Frodeno the guy who changed IM and the first super pro in long distance and Blu, … let´s give him some time to show where he has to be placed.
I do not want to disrepect Frodo, and yes, he won the olympics, but he did not succeed in ITU at all compared to Gomez, Lessing, Brownlee or even Mola. Hamish Carter won the olympics, was an exceptional athlete but he was no dominant as the Brownlees, Lessing or Gomez in short distance, so for me Frodo is not even in my top 5 in ITU (Lessing, Robertson, Gomez, A.Brownlee, Mola, Whitfield)