Whatever 100-grain (+/- 5 grains) gives good groups will quite well in a .243.
I use handloads with the Sierra 85-grain HPBT, although that load is now available from Hornady. It's a rather "blow-up" bullet, so I've limited myself to neck shots or cross-body heart/lung shots. No angling shots.
I've tagged a couple of dozen bucks with that load. Found 'em where I shot 'em.