The best one by far is called a notebook or diary, preferably solidly bound with a day to a page. They don't break down nor require you to record things in a particular way. Plus when you don't risk losing valuable data when you change phones or the app crashes.
There are a ton of 1RM calculators out there which are all reasonably useless. A decent rule of thumb is that the less experienced and the weaker you are, the more reps you can perform with a given percentage of your max. The problem with 1RM calculators is that they can grossly over or under-estimate your max. The best way to find out your max is to work up to it and see how much you can lift. You've also got to bear in mind a max lift relies not just on strength but mental approach.
IMO when starting out a good idea is to work on 3 and 5RM until your technique is good enough, don't even worry about your max single. Unless you're competing in powerlifting or something, arguably your 3RM is a lot more useful to know.