premium bullet
There was a time when the .243 was considered by some (including me) to be marginal as a deer cartridge. There are still some who hold that stance, but my opinion has changed after some use of the round with positive results.
As noted, the .243 had been a dual role cartridge since its onset, and for that reason there are varmint bullets and big game bullets for the cartridge. I suspect that was not fully understood by all when the cartridge was first afield, and may have contributed to the .243's controversial history early on. Whatever you choose, be sure the slug is intended for whitetails.
These days, more widely accepted and understood, the .243 is a pretty common cartridge for whitetails. That said, it is still a moderate/intermediate cartridge launching relatively light slugs (albeit usually at considerable velocity) when compared to other "deer" cartridges. For that reason, I use 100 gr Nosler Partitions, which is the granddaddy of premium bullets and available as a factory round from Federal and maybe others. The promise of additional penetration from the dual cored Partition is a comforting asset when angles are not ideal or a shot strays forward onto the shoulder or other larger bone.
I may well have seen more bang/flops with the .243/100 Partition than other cartridges, likely due to coaching bamaboy when he was younger and him shooting his granddads .243. Our sample pool is not all that high, maybe 10 animals between us, but we have not recovered a 100 Partition, despite punching shoulder blades, clipping the spine, centering the throat patch and severing the spine on a facing shot, and the boy connecting on one way out there at a lasered 260 yds through the lungs. All pass throughs.
The only .243 slug we ever recovered was a factoryW-W 100 PowerPoint, which caught the opposite shoulder blade, shattering same, and was recovered under the hide. DRT, but I feel a Partition would have exited.
You do not need a premium slug with bigger calibers, but I feel the .243 is a good candidate for the breed due to the comparatively lighter slugs it throws and its typical higher velocity. The Partition is likely the most affordable of the premium slugs and in .243 has worked for us.