I like 17 HMR, 6.5 Grendel, 300 WSM, 300 Blackout and 300 RUM.
17 HMR is a 22 with quality bullets. Good cost vs gain balance there, IMHO.
6.5 Grendel has great ballistics, but ammo cost is high and clear purpose is unclear...hunting??
300 ACC Blackout is a 7.62 x 39 for AR's. Super SHTF round, but not popular enough yet. I might get an upper for fun.
300 WSM - This caliber in a 26" barrel and 190 gr bullets is a thing of beauty. Any large 308 cal purchaser really ought to be asking why not the 300 WSM. It is ballistically king of the 300's. Bigger is excess. Smaller less useful!
300 RUM - super cartridge out to a mile. Good launch pad for 240gr plus VLD's. 30" barrel, weight, big scope required! A poor man's 338 Lapua!
I own the 300 WSM. Others have not moved me enough!