I do like your interpretation of the question, allthinky. 
And yes, Folkie, I agree - this question is a "Mission: Impossible" ...
I couldn't decide from SO many wonderful location choices, either.
Since recently watching I SPY on Familynet most evenings, I find myself drawn to and focusing in on the beautiful location photography ... talk about being
"transported away"!!!!
And I glaze over when seeing ALL those wonderful places one can travel to ... and, as Bulwer-Lytton has highlighted in her "I SPY Fan Manifesto," I do plan trips trying to see all these amazing places around the globe that Scotty and Kelly "adventured" in - as my (and others) THEN & NOW photos can attest to ...

)
But if I had to narrow it down a bit, I am totally mesmerized when it comes to the WAR LORD's hidden, exotic temple ... (supposedly set in Burma, but actually filmed in Spain), but who am I to quibble about a little latitude and longitudinal changes? Whatever and wherever it was,
that locale WAS magic to me and evoked every nuance and emotion it was designed to bring about.
I guess I am drawn to the Asian locations ... I adore the romantic waterfall and misty gardens of Toshugu shrine in TATIA, the bustle and exuberance of Hong Kong, the magnificent views from Victoria Peak and the floating restaurants and crowded marketplaces.
But then there are those amazing colors of the Greek episodes - the incredible blues of the Mediterranean waters and sky ... the ancient ruins of the Parthenon ... the sunbaked white-washed houses and walkways of Mykonos ... they certainly provide a place to take your breath away ...
And we can't overlook the sights and sound of Sevilla and the magnificent Pavilion buildings Kelly strolled along with Boris Karloff .... and then the Mexican episodes with the intricate gates and beautiful gardens at
Chapultepec Castle in Mexico City, and the colorful boats in the
Floating Gardens of Xochimilco in BET ME A DOLLAR, and the Pyramids of the Sun and Moon from CRUSADE TO LIMBO ... and THEN there's Venice ...
O.K. I give up!!!
I can't do it!! There are just tooooo many wonderful places that Kelly and Scotty
and Sheldon and Earle and Fouad took us to - for me to pick a favorite!!
Let's here from some others of you ... what are "your" favorite I SPY locales???
~ ~ ~ ~ As always,
~ ~ ~ ~ Tatia
~ ~ ~ ~