I've had all these rounds mentioned except the 6 arc, but had the wildcat versions of it which the arc basically copies.
6x45, easy, cheap, accurate, was just uninspiring for some reason. Handles up to about 87 gr non tipped bullets pretty well.
243 lbc, very accurate, more expensive Grendel brass but not terrible. Weak bolts so buy a good one, don't cheap out as I broke cheap bolts, 2 of them with a round count of about 1400 rounds iirc. This was basically a 6 arc. I shot 87 vmax mostly at 2900 fps or so iirc with 8208.
243 wssm, very accurate, very tough brass, 87 vmax at 3200 from a 22" barrel. Proprietary parts other than the float tube so you'll have to have one built if you go this route. But it's a coyote hammer! Obvious reduction in barrel life from the others so it's not a daily driver. And mag capacity limit of 7 or 8 I think. Not a problem for me but worth mentioning.
243 win, I've had two, the first built 8 or 9 years ago on a regular 308 platform and used a cf float tube. Somewhat heavy and awkward, was accurate. Took it apart and sold off the parts.
Second 243 win I bought a year ago, dpms g2, one of the last ones I've seen for sale. Sweet, sweet rifle imo. If you're not familiar these are almost a cross between the 308 and 223 sized platforms. It carries like an ar15. Uses a bolt carrier about 1/2" longer than a 223, and smaller receivers than the old 308 stuff. Carbon fiber float tube and 20" barrel. My load with 87gr bthp and H4350 is 3070 fps so a big jump from the arc but noticeably less flat than the wssm. Most importantly though this rifle might be the most consistently accurate one I've ever had, with 200 yd 3 shot groups never over an inch, mostly about 3/4". And shot out to 700 it stays 1/2 moa. The best part is cdnn was selling dpms barrels for it in 243 and 260 for 80 bucks so I picked up a few. As long as I don't break one of the other parts I'm set for a long while.
As far as long vld type bullets, the wssm and 6x45 are out for sure. The 243 has a 9 twist so limits it somewhat, my wssm has an 8 twist but you run out of mag room way before you get to the 108 type bullets. So that really only leaves you the Arc if that's the bullets you want to run.