I learned the original version of The Entertainer three years ago (as said above so : http://ks4.imslp.info/files/imglnks/usimg/8/89/IMSLP130674-WIMA.e04e-Joplin_The_Entertainer.pdf
It is difficult. The left hand movements (stride) isn't the main problem. There are some little sections (one or two bars) really very difficult.
Mapple Leaf Rag is way easier despite the number of flats.
But it is fun to play, and you learn a lot.
Speaking of pop songs, there is no level, strickly speaking, because you will only find transcriptions scores. Pop songs are not written for the piano. Even if it's a pianist singing alone. The best scores you will find generally is three staffs that must be reduce to one. And the pianist, while performing on stage, plays what he wants (no score).
In order to have good scores and level indication, I suggest this site : https://www.noviscore.fr/
(I don't have any price and pay as anybody...)
There are 4 levels : 1 to 3 and expert.
You can listen the scores.
I always choose the expert level (or 3 if there is not). Because for the same price you have more music work for your money.
For example, look at the expert version of "My baby just cares for me" : https://www.noviscore.fr/partition-piano-my-baby-just-cares-for-me
(a pleasure to play)
This is not singer and piano, it is a transcription for piano solo. Very difficult to find and very difficult to find well written.
I worked also on https://www.noviscore.fr/partition-piano-georgia-on-my-mind-ray-charles
it is delightful. The transcription is fabulous (for my ears)
Just for fun I work also on https://www.noviscore.fr/partition-piano-when-the-saints-go-marching-in-gospel-traditionnel
You must listen to it ; choose expert level (wait untils it begins really...).