I probably don’t know enough to have a good opinion, but I love Excel. I use it all the time at work and at home. It’s so useful.
Saving those 4 characters mattered back in the day…
When BASIC was an interpreted rather than a compiled language…
Telemark nominated Unix kernel, but Unix usually comes bundled with a C compiler and utilities. If you treat Unix (and its derivatives like Linux and MacOS) as kernel, compiler and utilities collectively then it is an easy choice for #1 greatest!
We have frequent threads here for “How do I do XXX in Windows?” where a Unix solution would be trivial. The ascendance of Windo$e is a tragedy. 
True, but credit should be given for the concept of writing an OS in a HLL. By the time of UNIX it wasn’t all that controversial.
Maybe UNIX should get credit for being effectively open source before that term existed. It wasn’t the only one - I did my dissertation on the Jensen and Wirth Pascal compiler which I had no trouble getting the source for.
And it was invented at Dartmouth. My now wife was a grad student there and I used the Dartmouth Time sharing system when I visited. It was pretty advanced for 1976.
Let’s not forget David Ahl’s book of BASIC games which I got the famous Star Trek game from in 1974, which I rewrote in Pascal for a PDP-11.
I used Microsoft BASIC on the C64 and it was pretty good, but not earth-shattering. And my CS professor friends at the time dreaded the freshman BASIC programmers who were convinced they knew all about programming. shudder
I’ll go with the code that ran on Colossus at Bletchley Park, deciphering German Lorenz messages. It’s credited with shortening the war by many months and saving countless lives on all sides.
COBOL wasn’t first, but it was more influential.
Regards,
Shodan
This video from Nostalgia Nerd would imply that Lotus 123 would be in the running as it forced people to upgrade the resources in their computers that later programs would take advantage of.
Personally I would go with WordPerfect and if forced to choose a specific version I would choose WP 5.1. While the spreadsheets were good, it was WP that was the first to change our fundamental understanding of the computer as a tool and shift our concept of how to do something. Before WP everyone used a typewriter; after WP everyone used computers.
That’s a specious claim that seems to reflect nothing more than your bias for business computing. In the scientific realm, FORTRAN was absolutely dominant and remained so even after many newer languages were developed. Even the very early IBM research document I cited regarded FORTRAN as a general-purpose method for writing programs that could be compiled into efficient object code. Despite its name and its mathematical orientation, FORTRAN was indeed a completely general-purpose language. In the milieu of academic computer science departments, COBOL was frequently the butt of jokes and ridicule. Not that it wasn’t a very important language for business applications, it was just so plodding and inelegant. FORTRAN was used to plot spacecraft trajectories to the stars, while COBOL was perfect for producing the print layout of your electricity bill. 
Unless it was a Monday. Garfield won’t do anything on Mondays. 
Only if you were in business. I’d say the most influential language was Algol60, which was the ancestor of most modern languages in terms of syntax, including Pascal and C. Not many in the US used it, but we all knew about it.
COBOL on the other hand has no descendants.
I was in 3 CS departments during my academic career, which was during the prime time of COBOL and I don’t think any of them taught a COBOL class, and if any of the professors knew COBOL they were ashamed to admit it.
COBOL is the programming language for those scared of math. 
This.
Or Android.
I thought they were all tokenized anyway, but I may be wrong. Or maybe in just some implementations. Actually, now I don’t know for sure. I seem to remember on a VIC-20 if you typed in ? “HELLO!” when you LISTed the program, it would show up as PRINT “HELLO!”
I’m an ex-CS prof and seen a ton of important software over the decades. Visicalc, Linux kernel, Xerox Alto word processor, etc., come to mind. But for me it comes down to one thing:
Mosaic.
I had been using various Internet tools but this was The One that made me go “Wow!” I knew right away it would change everything and it did. Nothing else ever made me go “Wow!” like that. Not even close.
Would Android be under consideration or at least on The Top Ten list? Or is there another program that lead to smartphones becoming the dominant market for cellphones?
According to this link Android phones consist of 74% of the smartphone market.
Purportedly, it would be NLS from the Mother of All Demos, which preceded Xerox PARC:
This is an awesome answer. Don’t know if it’s the “greatest” program written, but I had no idea about this and, given your description and its impact on the complexity of software which could be written, I could definitely see SPICE on a “top 100” list.
Which is why IMO it is more influential. There are a lot more business applications (I would guess) still running COBOL - I have maintained some of them myself.
How many spacecraft trajectories have been plotted, vs. number of electricity bills printed?
Regards,
Shodan