Which ninja warz relic?

lvl 20 with 17 ninjas and 6 relic slots. currently using blood sphere gem, chaos emeralds, sushi roll, genbu's scale, and team doushi. wondering which would be best for the 6th slot: craven rock (30 power per ninja), dianthus flower (10% health per ninja, 5% critical), bloodshot button (10% health, 10 power per ninja), or black opal shard (5% health per ninja, 4% speed, 5% critical, 10 power per ninja)? craven rock is obviously most offensive and black opal shard is most balanced but does the more specific stat increases of the dianthus flower and bloodshot button make them more worthwhile? more specifically: is losing the 4% speed and 5% critical worth the extra 5% health by substituting bloodshot button over black opal shard? or should i dis regard the small increases to health and such and use the craven rock for the extra power?