there’s really no such thing as a best super art for ryu. all three is great in different ways, IMO.
denjin doesn’t offer as much meter for EX moves as shinkuu. shinkuu doesn’t offer as much damage as shinshoryuken. and shinshoryuken is harder to land compared to denjin.
denjin allows more room for creativity, that’s why i like it =P but sometimes when i’m struggling i use shinsho =/