If we're listing 'other' calibers: 5.45x39, 22Nosler, 224 Valkyrie, 6mmx45, 6.5 Grendel, 6.8 SPC, 300AAC, 7.62x39, 9x19.
The least satisfying/most finicky is w/o a doubt 6.5 Grendel.
The best deer hunting round is the 6.8SPC.
The one with the most "cool factor" is 300AAC sub-sonic through a suppressor.
The best "bang for the buck" ammo cost wise is the 5.45x39( I have a lot of ammo that cost less per round than 22lr does these days).
The Nosler and Valkyrie are what I'd call 'specialty rounds' that approach 22/250 performance from an AR 15 platform.
The 6mmx45 came here as a kids' deer rifle and it's done well in that use parameter. I wouldn't consider it a general use round due to it being a handload only item.
The 9x19 is just an odd duck that may require a fair amount of fiddling/tuning to function properly. Projects considerable power at short range w/o the ear shattering muzzle blast of a rifle cartridge in 8.5-10.5" barrels. It's even better with a suppressor and sub-sonic ammo.
The 7.62x39 niche is that I already had several rifles in that caliber and a good supply of the ammo. My first one was a fairly rare model and when a collector offered me a new 7.62 upper and 10-12 magazines in trade, I jumped right on that deal. A 16" 7.62x39 makes an adequate 150-200 yard deer rifle with the right ammo. My latest 7.62 addition is a 10.5" 'pistol' upper that is surprisingly accurate.