First three long guns for someone starting out thatI would recommendare:
a 22LR, a 12 gauge, and a 30-06.

When your rack of rifles is limited, thena 30-06 is a great asset due to its versatility.

For those who believe in cartridge spacing,and go about collectingadditional rifles, thenthe 30-06 will tend to become less and less valuable due to its "jack of all trades master of none" characteristics as it compete with ones more specialized rifles. Eventuallythe 30-06can bereplaced unless you have got real attached to it along the way.

If I was phasing out an '06 out of my rack, I'd replace it with a 308 for target work (already got that one)and a 300RUM (looking real hard)for hunting (same caliber, but very different cartridges). Probably won't get rid of the 30-06 since I like taking a 30-06 along to elk camp as one of the back up rifles in case someone in the group has a mishap --- anyone can shoot a 30-06 without recoil management savvy, the mags generally require a little more cognitionon the part of the shooter.

Per Stubblejumper, I would also agree that a 7mmRemMag is a good 30-06 equivalent/substitute depending on your tastes and preferences. Of course it does have the dreaded "magnum" word in its name which makes it automatically unmasterable to some.