Except for the Pentium, I can tell the age from the number. The i3 is probably the fastest, but since stuff uses less power these days, there's a small chance the old i5 is faster. I'd look up benchmarks if about to make a purchase.
Is the Pentium equivalent in age to 7th gen core i, or is their numbering different?
These aren't realistic choices unless you are buying a used PC. But even less savy consumers understand generations, if you present a fast 1st gen model versus a not the fastest 11th gen model I think the majority will go with the newer model.