Open ended question obviously because it all depends on what you need to use the gun for. Hunting, personal defense, home defense, ccw, competition, punching holes in paper etc.
That said, I'll give a list of my favorite modern (smokeless powder) guns for overall quality, reliability, inventiveness, longevity, popularity. In no particular order:
* Colt 1911 45 automatic by John Browning - military sidearm for 50+ years. as effective and more popular than ever today, almost 100 years after its invention
* Winchester 70 (pre-64) in 30-06 (or 270 for you Jim Carmichael fans) - the riflemans rifle can do just about anything you'd ever need it to
* Savage 99 in 300 savage - possibly the strongest action in a lever action ever. Pointed bullets could be used, hotter cartridges accomodated, slick action, magazine fed rather than tube fed. I'll take one of these over a Winchester 1894 any day.
Ruger 10/22 - Bill Ruger designed the best 22 ever, IMO. the magazine alone is an engineering marvel.
Remington 870 shotgun - still going and still hangs with shotguns 10 times its price. Can be used for everything from sporting clays to home defense.
AK-47 - I know this forum is full of AR-15 fans and it is amazing. But the Ak-47 has been around longer, has been more reliable (and still is), is significantly cheaper and is used by many more people worldwide. If you are looking for the fastest, cheapest way to put effective military firepower in as many hands as possible, the AK-47 is unsurpassed.
Right now, if I could only have one gun. I would choose my Sig P226 Elite. Accurate and reliable as they come. better than any Glock, 1911, XD or Para I have owned and fired. Best feeling gun. Best DA/SA handgun trigger.
If I had to have one rifle, I would pass up the urge for the very practical AR-15 platform and get a Remington LTR in 308. Long range precision accuracy coupled with short range maneuverability.