there are so many different answers here..... Which calibers do you own ? I suppose maybe a 30-30 or a 45-70 which are both good but not quite the "fast" bullet pushers.
I would go for something that you have always wanted or always heard of people talk about it. a 270 is good but so is a 7mm mag or a 30-06 or a 308 or a 280. This is the Medium world
Then you have the smaller world of 223, 243, 6mm, 6.5X55, 257s and such.
Then You have the super fast magnums of the 30 caliber and 7mm family 300 win mag, 300 Rem ultra mag, 300 wby mag, 7mm RUM, 7mm STW, 7mm wby mag etc (those are not so fun in my opinion, soo fast and provide no real advantage over thier standard brothers)
Then you get into the medium & large bores like the 8mm rem mag, 338 win mag, 375 H&H, and..... anything bigger is just rediculous.
So its all about what "you" want and grabs your attention the most