Best App to master chess

ELO ratings are calculated differently by different organizations & as for mastering chess tactics I think playing against as many human players as possible or playing against a good AI will help but I don't think you can get detailed info about AI chess engines behind all those chess apps(if using any). Just go with most popular chess apps with good ratings.

Yes, ELO rating is different but the creator of this concept was one single person.

The Chess champion uses FIDE rating worldwide though.

I just download Chess Tactics Pro. Lets see how it goes.

Any experienced Chess Players in TDF ?

I don't think I have ever seen experienced chess players here, though I also used to play chess but only casually.