What is the best brand/weight grain to use for my 243?

The best brand is the one that will shoot a 100-grain bullet most accurately in your rifle. Or you can get into handloading and work up an accurate load to use.

You need 100 or 105 grain bullets for deer, though there are some 90 and 95 grain bullets designed to allow 244 Remington owners to use their rifles for deer hunting. The lighter bullets, including some of the 90 grainers, are designed for varminting and should never be used on deer.
The Remington load is OK. It may fail (blow up) occasionally on a close-range shot, but that’s true of all the non-premium spitzers in miniature calibers at high velocity. You’ll simply have to try it and some others to see if your rifle likes them. It’s the normal thing for rifles to prefer one load over another, and it’s completely unpredictable. If you can afford premium loads, I’d trade some accuracy for a little more certainty that they’ll set up properly. 2-minute accuracy with a bullet that’ll do what it’s supposed to is better than 1-minute accuracy with uncertain terminal ballistics, but that’s up to your pocketbook.

Dude if it ain’t broke don’t fix it. My wife has been using remington Exp. Core-Lok and has taken many deer with it my only advise is to go with the 100 grain for a little more damage to be done upon impact.