This app was mentioned in 1 comment, with an average of 7.00 upvotes
~~Someone~~ /u/Fluttertwi in a /r/SFGiants GameDayThread was asking for recommendations and I outlined what you should look for as well as ~~my suggestion~~ what I like. Check out my comments there.
I'm also going to add that it should be wider than it is long, as a personal preference.
At the moment, I've been using my personal notebook/cookbook/lifting-log/TotalWar-reference-guide that I carry around with me (It's just a hardcover 5x8"ish notebook that I got from Office Depot) and playing around trying to develop my own system. It's a mix of the Reisner method, traditional scoring keeping, and baseball-reference play-by-play log.
Here are my attempts to refine it. It falls short of my recommendations, I know, but I carry this notebook pretty much everywhere. I'm thinking of carrying post-it notes because they're awesome and can solve my problems of pitchers list and >1 substitutes.
I've also tried several free mobile/tablet apps. Scorefinger I liked the best. Ultimately, on mobile, I think a drawing app that allows me to zoom in and have very narrow pen thickness suits best. That way, you can just download a scorecard you like as a pdf, and just save-as each game as you score it.
TL;DR: To be clear, my only suggestion and strongest recommendation is that you find one that suits your need.