If you are looking for both capacity and performance, I highly recommend the Sandisk Extreme series cards (not Ultra). I went through a great deal of frustration a while back with seemingly random pauses on ISO based games due to either transfer rate limitations or more likely interruptions to the data stream from the ISO image while playing a disk image based game. I finally found what I consider to be the best solution for both speed and capacity in the Sandisk Extreme 128GB SD card. Once I found this card, the issues I was having all but vanished and I've never needed to test another card. The only downside is it is pricey, so you might want a smaller card if you don't need the capacity. I should mention I have not tested smaller cards in the Extreme series though. In the thread link below, you will find my final post on the subject at the bottom with benchmarks.
https://retropie.org.uk/forum/topic/4288/sd-card-experience-requested-fastest-128-gb-card