"... to be a great collection" is a really open-ended question.
I'd say a 22 rimfire rifle and handgun, a 12 gauge pump, a bolt action centerfire rifle of with decent power topped with a scope, a 30-30, 44 Mag or 45 lever action, a 4" barrel 357 Mag revolver, a go anywhere carry gun, a 9mm pistol and a 1911A1 should be on almost everybodys list as should something that shoots .223, a 7.62x39 or a 7.62x54R.
And of course, everyone should have a Garand. A 1903, an Enfield, Nagant, a Thompson, M1 Carbine, a bunch more military rifles, a couple of Mausers, a Borwning HP, a P08, a P38, a Makarov, Webley and a few more will make a pretty good collection. Still not great...