I never owned a .243 in my life until I just got a good deal on one. Used to using .300 Winnie, 30-06, and 8mm mauser. So what is a good bullet and a good load for deer.

100 gr Speer Grand Slam, and Midway has them on sale for 5.49 a box.

Midway # 236909

22WRF
My wife and I have taken sevral dear, and she has taken several antelope with 2 different 243,s.
We used either a 100gr Sierra or a 95 gr Nosler Partition. I loaded 40 to 42 grains of IMR 4350 with either bullet.
When hunting deer in CO or deer and antelope in WY we used the 42 grain loads. At the deer lease where shots were fairly close we usually used 40 grains.
Start about 38 grains of IMR 4350 and work up.
My wife killed some big deer and made some long shots with her 243. In our experience it kills deer [whitetail and mule] and antelope as good as anything else.

Bullet: Sierra 85 Grain HPBT
Powder: 40.4 grains of IMR-4350
Primer: Remington 9-1/2
Case: Remington
Firearm: Ruger 77R
Optics: Leupold 3x9
Velocity: 3010 FPS @ 15' from muzzle
Accuracy: 5 shots, 200 yards, <0.75"