A class 10 card that writes at least at 40MB/s is best if you plan on taking RAW photos. The Phantom only writes 4k video at 7.5MB/s, so most any class 10 card will work. You can find some commonly used memory cards here:
Any Class 10 (or U1) marked card will deliver 10MB/s sequential write speed. 4K ad UHD video is recorded at 7.5MB/s (60mbps encoding rate).
The P3 camera is limited to SD bus "Normal_Speed" (12.5MB/s) so buying a faster card will not help you take photos faster. The only thing a faster card will do is make it quicker to copy the data off the card onto your computer.