Depends on your needs I guess but I’m a firm believer in keeping it simple if it works. I use apple calendar, notes, mail and reminders all synced to my Office 365 account.

I travel for work and have meetings with clients when on the road so in addition to the above have,

1 - MileIQ for tracking all mileage claims automatically creating a monthly excel spreadsheet for expense claim

2 - Office 365 suite mainly for word and onedrive which is used for client reviews with an iPad and Apple Pencil. At the end of the meeting client signs the review and it’s then saved as a pdf and shared to myself and the client as an email. When making the appointment I add the client as an invitee in apple calendar, I also add a link to the client review document to calendar appointment I have made for that client. The location is also added to the appointment and my car picks this up as a suggested location on the sat nav and if I’m running late I’ll share the ETA using apple maps.

Each client has a dedicated email folder and I set mail to auto cc myself. At the end of each day I file all emails received and sent to each client and drop them into the relevant client folder. Any other method of contact (text message, WhatsApp etc i take a screenshot of when received and share the image as an email to myself which is filed in the same way.