It's a very, very large open question you are seeking an answer for.
What the bikes intended use is means a lot. I am going to answer in broad strokes:
It's hard to go wrong with a 34-50 crankset with an 11-32 cassette for a general purpose road bike with moderate terrain. With practice you should be able to clear mountain passes with out issue.
If your terrain is flatter or you are stronger an 11-28 would work well. More cadence friendly still would be a 12-28 cassette.
If you are super strong or ride exceptionally flat terrain an 11-25 cassette would really let you dial in any cadence you want. Mate that to a 36-52 crankset or 39-53 if you are a professional riding in pancake flat Florida with a strong wind at your back.
Going the other way: It's not uncommon to mate a 34-50 or smaller crankset (or single ring) to an 11‐34 or larger. I think cassettes go up to 50-ish now. If you have super steep climbs or are not strong this may be better. It's terrain & fitness dependent.
I had a 28-44 crankset with an 11-40 cassette. I needed an XTR mountain derailleur and a pull ratio converter from J-Tek to make it work with 11 speed road shifters. The range was from 19 to 109 gear inches...After a year or 2 I decided it was too broad. So I changed it. There just wasn't enough times I needed to carry 75 pounds of camping gear up 20% grades to justify the huge jumps between shifts.
If I were to build another city bike, I've had good experiences with an internal hub like a Nuvinci N380 and also Nexus 7 speed hubs with an 18 tooth cog mated to a 36-52 crankset.
In all cases, I find a range of about 28-32-ish gear inches at the low end to about 90-100 gear inches works for the riding I do in the terrain I tend to ride.
So grab an online gear calculator & see if you can get a feel for what will get you in that range.
If you are getting a mountain bike, anything less than 15 gear inches is beyond utility unless you are exceptionally talented with balance, power delivery, and high traction terrain. I'd call 15 gear inches the lower bound for a mountain bike. But others may disagree.
Last edited by base2; 01-07-20 at 02:54 PM.