When I had my first digital rebel (the original digital rebel, eventually named the 300D, I believe) I purchased a used Canon EF 28-70 f/3.5-4.5 II. Although old, this was a great lens on a crop camera. Some reviewers consider it as sharp as many Canon L lenses of the time, and not only was it my favorite lens at the time, but it cost me less than $100 if I remember correctly. When I went to FF after my rebel bit the dust after 9 years, alas, I sold the lens as there was no lens correction profile for that lens and the vignetting was pretty bad on FF. In all other respects, I think it had equal IQ to the EF 24-105 that I got to replace it - and of course, the 24-105 has a much more practical zoom range. I still miss that 28-70 sometimes!
Just recently, I bought a nice used Canon EF 24-70mm f/4 L. So far, it seems to be a really good lens, and though I haven't has a chance to use it much, I'm intrigued with it's "Macro" mode. Paired with my Canon EF 16-35mm f/4 L, I have some nice EF lenses to use on my R6 camera.
I've tried a few others that I liked, but didn't keep for one reason or another. I really liked the IQ of the Canon EF 70-300mm f/4-5.6 L, but just couldn't handle the weight.
Hopefully people will keep selling of their EF lenses, although since the release of the R5 an R6, many EF lenses have been harder to find used and the price has gone up a bit, but, in my opinion, they are still a great bargain compared to the RF lenses that are available.