1. .30-06. Sure, nowadays, .308 is the preferred .30-caliber round, but I have to go with the '06 purely for the historical factor. It's a great round for hunting, but it takes on a new significance when put through a Garand.
2. 5.56mm. Having spent eight years as an Army infantryman, I have developed a relationship with this round and the AR15 platform like I imagine many other current and former servicemembers have as well.
3. .22LR. Cheap, fun, accurate, cheap, fun, fun, cheap, accurate...I could go on.
4. .308. Good in a rifle, good in a machine gun. It can go the distance and it hits hard, nothing not to like.
5. .50 BMG. Making 90% of all forms of cover obsolete for the better part of a century.