What's a good brace for a possible ACL or meniscus tear?

None. There are no braces that can substitute for the functions of an acl. Bracing technology has advanced greatly, but most of the rotation stability needed is not possible with external braces. Most braces provide stability in side to side instability helping mcls and lcl (limited). Pcls are also partially protected with bracing. Meniscus tears are also not protected.
Hinged brace. A hinged knee brace, commonly sold at sporting goods stores, may give you some support. A fitted ACL brace would need an rx from your doctor, and you'd probably have to get fitted for it.