The best replacement for 243 is 308.

The best alternatives to 243 are rated as following:
★★★★★  308
★★★★  270
★★★★  223
★★★  22-250
★★  260
★★  30-06
★★  6mm
★★  7mm08
★★  25-06
★  25 06
★  257 roberts
★  799 heads
★  22 250
★  7mm 08
★  30-30
  243 wssm
  7mm
  6.5 creedmoor
  308 ballistics
  317 heads
  7mm-08
  270 rifle
  270 recoil
  308 recoil
  30 30
  25
  241 heads
  30 06
  6.5 creedmore
  6.8 spc
  .270
  6.8
  6mm br
  257 weatherby
  6.5
  6.5x55
  270 caliber
  260 remington
  22 250 ballistics
  223 ballistics
  3030
  25 06 ballistics

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Speaking about 243, one probably shouldn't forget about 1 in 8 twist.

Other search terms related to 243 and their ratings are:
★★★★★ 1 in 8 twist
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