Hello!
Personally, I use the "Thank You" Image. However regarding the "Sub for Sub" method, if you don't feel like subscribing unless you like it, maybe just visit their series then like & comment on their series a bit? You know, show them some love!
I would suggest you use the "Thank You" Image, but leave a bit of a personalized message, it doesn't have to be drastically different, you could just simply use their username to address them, or wish them luck with their series if they have one? If they don't just say you hope they'll enjoy the series?
I hope this helps! 