I've been sticking with Sandisk ones. I've done about 7 total 3DSes using Sandisk cards and they've all been without issue. You'd want to make sure it's a Class 10 card or higher. As far as sizes go, I'm using a 64gb without issue. I've heard 128gb can have issues but I can't verify
Since the 3ds hardware was only made to support SDHC, I only use 32Gb SD cards but there are reports of people using SDXC (64Gb and up) with their consoles but they have to use third party programs to format the card in FAT32 with 64Kb cluster size.
For brands, if buying from Amazon, any Kingston, Sandisk, etc will be Original and not a knock-off.