saving your seed on any kind of mobile app or note on a PC/Laptop is absolutely not advisable.

I personally feel the safest is a diary, but I have had people have an opposite view on this.
but a diary is way better than a piece of paper, I'm sure you have faced the issue of finding an invoice or some paper on which you have written a phone number and now can't find it.
I keep my diary really safe so if you are that kinda person, I would suggest a diary over a piece of paper and please write using a BALL Point Pen