Thanks for the replies. Some good points made. It seems my question was misunderstood.
If your goal is to help someone to have fun and enjoy shooting, I would say red dot. Irons have their place, but if I just had an afternoon with someone and wanted them to leave enjoying the experience I would give them a .22 with a red dot and fun targets.
Might be a different answer if you are teaching for something else or have a captive audience you are training.
It's kinda the second one. As I said in the poll question, "teaching rifle marksmanship basics to new shooters." That means breath control, sight alignment/sight picture, trigger squeeze, stance, etc. I think that's usually different from taking someone shooting for the first time, or trying to get them interested in shooting. I'm asking about someone already interested, and now they want to learn how to go from barely getting on the paper to getting more respectable groups. Maybe I should have spent more time explaining what I was asking about.
I guess some people are reacting more to the thread title, which is a less specific question. For thread titles, I often sacrifice a little accuracy, in favor of something shorter, and more eye-catching. Sometimes that doesn't work out so well.
Your question certainly implied exactly that.
I mentioned 50 and 100 yards as limits (not as starting points), because I thought that would keep the focus on entry-level marksmanship, and not privilege magnified sights too much over the irons and red dots. I agree that 25 yards is a good place to start with rifles. After that, wouldn't you go farther out, to see if they're really developing the skills of a marksman? It's a rifle, after all.
I hope that clarifies things a little bit.