"Classic" does not mean rare, odd or unobtainable. It means "of enduring interest, quality and style" (among other things) and "serving as a standard or guide."
With this in mind:
1) Colt single Action Army in .45 Colt. Introduced in 1873 and still going strong.
2) Colt 1911 in .45 acp. Introduced in 1911 and more popular than ever.
3) Smith & Wesson Model 19/66. The epitome of the perfect marriage between handgun and cartridge.
4) Smith & Wesson Model 29/629, preferably with 6 or 6.5" barrel. A handgun/cartridge phenomenon that's never been matched.
5). Glock 17. It redefined combat/practical handguns forever.
Oddly enough, I currently have all of these guns and I would agree wholeheartedly with this list. If these were all you had, you would definitely have all your bases covered. Probably the only gun I would add is a Python, discounting how expensive they have gotten lately, they are both beautiful and functional. I was lucky enough to find one before the prices got crazy, and even though I could turn a tidy profit, I'll never sell it.