Hmm, 10 must haves? Some of mine will be categories and this will still be tough:
#1 (tie) 1911. Right now I only have a Commander sized S&W 1911SC (I sold my Charles Daly), but I would like to add a full-sized again shortly after I start buying handguns again.
#1 (tie) S&W K-frame .357mag (of your choice). Right now I only have a S&W 65LS in this category, but I'll add more (again- never should have sold some of those K-frames).
#3 SIG 250 or 320: I like the modular design for flexibility, your choice of caliber or size for the day, same gun...I like the compact and subcompact for carry. If you hate the modular design, I'd say any compact carry gun in a service caliber is a must have category (SIG P239, Glock 26/27, Taurus Millennium Pro, any number of compact or subcompact models of larger guns, etc).
#4 Small easily concealable micro 9mm. My votes in a specific gun would be Kahr PM9 or SIG P290RS (I voted P290RS with my own money, though I may add the Kahr later).
#5 Small framed snub revolver: If you are buying to use as a CCW gun, whichever of the major brands appeal to you should be a good choice (I have both S&W and Taurus, a Rossi, and will add a 3" SP101). If you are buying just to have an example of a classic carry gun but not necessarily to carry, it has to be a S&W or Colt.
#6 Single Action revolver. If you are a collector type, it kind of has to be a Colt SAA. For those of us on a budget (or who will shoot the snot out of it), any reputable brand will do.
#7 Blackpowder revolver replica. I love my Remington 1858 New Army and Colt 1860 Army replicas, they are so much fun to shoot.
#8 Classic full size 9mm. Mine is a CZ 75B, but I also need a BHP.
#9 Walther PPK in .380 or similar. For years it was the only real choice if you wanted a deep concealment gun in a decent caliber that wasn't a snub revolver. It is a classic. Funny though, while I consider the PPK a near necessity for a well rounded collection, I don't really want one, though I used to. The closest I'd actually buy is a Beretta 84 in nickel, a CZ 82 (though in 9x18) or a CZ 83 (preferably in nickel).
#10 A .22lr for cheap shooting. Right now I'm without a gun in this category. I never grew to love the Ruger MkII I had, I'll probably add a .22lr 1911, a .22 version of a service pistol I own or want to own (CZ Kadet kit or add the .22 and .40 version of a SIG P229 or .22 and .45 version of the P220), or a revolver (probably a SA).