I really like
Darkr for black and white conversions on the iPad. I like the results I get from this app better than what I get from others, and it is fun to use. It visualizes black and white conversion of a color image as developing a physical negative into a print in the darkroom. It sounds kind of corny (and maybe it is), but it actually results in a very usable interface with granular control over global contrast and easy to use dodge and burn tools. It works well with RAW files too. It is not quick to use, however, and I would not want to have to process a large number of files with it on a deadline. It's a rare app that is still free with a one-time purchase of $3.99 to unlock all the pro features. I did that so long ago that I do not remember what features are not included in the free version.
Snapseed is pretty good for black and white conversions too.
[Just want to say up front that I am mostly a film shooter and my knowledge of processing digital images is, I am sure, only a small fraction of that of other people posting here. I just do this stuff for fun, and these apps work well for me.]