Here's my list of replacements, starting with the one you asked about.
Contacts is replaced by Playa Apps' A2Z Contacts and Synctastic
Calendar is replaced by Readdle's Calendars and Calendars5
Calculator is replaced by Creative Creek's MathU RPN Calculator (emulates an HP RPN)
Notes are stored either in KeePass Touch, Google Keep, and gTasks Pro, depending on what they're for.
Maps are replaced by Google Map
Mail is replaced by Google's Inbox(Personal) and Readdle's Spark(Work)
Videos, Music, Podcasts are replaced with VLC for iOS. I don't need iTunes to load it.
Reminders are replaced by gTasks Pro, which does tasks, reminders, location alerts etc. etc.
As far as Wallet, FaceTime, Tips, Just Friends, Watch, and News are concerned,
they're in don't give a damn status.
Health might tempt me except it apparently insists on reporting everything to Apple's cloud.
As a result, I wouldn't touch it with at a hundred yards with a waldo.
And Game Center is an abomination that apparently wants to push me into being "more social." Meh.
FYI, I use Marvin 3 and Marvin as my fiction readers, Hyphen for fact books/professional books/science articles.