As phrased, it's not a very useful question. In other words, which program are easy to earn some points/miles in (without staying/flying) are not the same programs that are easy to earn meaningful amounts of points/miles in.

Your own example of IHG is perfect. By your original question IHG qualifies, but if you rephrase for meaningful amounts, does it qualify?

Yes, but only if you include credit card signups. There are many programs where the only way to earn meaningful amounts of points/miles are to do credit card singups (perhaps more than once over sufficient time), and/or to hold a particular credit card.

For example, holding the Chase IHG Club Rewards card for more than a year gives you a free (uncapped) hotel night certificate in return for paying a $49 annual fee. That gets you a free night that might have taken many tens of thousands of points. Could you have gotten anywhere near that many points from the "free" (non-credit-card) methods you mentioned?

And signing up for that same credit card gives you 40k or 50k or 60k or 70k (or however many) points, depending on the signup offer (with some minimum spend in a certain period of time). That dwarfs the tiny amounts you found you can get free to the point where it makes me wonder whether it's ever worth bothering with the "noise" of such free offers which give so few points.

There are plenty of overseas airlines and overseas hotel programs that don't have a US credit card, may not have US-based partners, etc, and so any of those by definition are among the hardest to earn for someone based in the US. There's just many of to list beyond naming them as a category. (Just one example though: Scandic Hotels used to be affiliated with Hilton, and when they broke apart, Scandic gave me their top level tier status but no meaningful points. I had no way to earn meaningful points with Scandic while based in the US, and my status over several years evaporated, only level down a year, and still last time I checked they don't have any partners based in the US where I could earn Scandic points.)

Meanwhile, I'd say Delta is easier to earn in, because in addition to their credit card, they are the only major domestic airline that participates in Amex Membership Rewards, and there are lots of cards that earn those. Yes, you can wonder whether Delta is worth redeeming with, but again that's a somewhat separate (though valid) question relative to what you asked. But be aware that who is worth redeeming with can change from to year; I just booked a fee-free business clas trip LAX-AMS-LAX for just 125k DL miles for next spring, whereas UA would have wanted at least 140k (because only partners flights were available), and AA/OW only had BA with their super-high YQ fees. (I was not booking 11 months out, when there may have been better availability on other airlines, but I was booking 6+ months out.)

With every trip I plan, I seem to be surprised by who has what availability, because it keeps changing all the time.

And what's easy to earn keeps changing too. Until earlier this year, Hilton HHonors were by far the easiest to earn because it was possible to "quickly" churn the Citi HH no-AF Visa for 40k/50k/60k every few months. It's HHonors that I'm thus "drowning" in the most. To the less degree I'm "drowning" in with IHG, it's not due to any free ways of getting points, it's due to the fact the I earned so much in "Crack the Case" and similar promos (where I needed very few stays to earn tons of points) and then found very few opportunities for meaningful redemptions since then.

And finally, what program is easy to earn in but may be a while before you can redeem in and yet expires too quickly can be useless. So all that's discussed in here has to be balanced with your other thread (in MilesBuzz):