UTC, always use UTC

Although it is possible, don't change the timezone on your App Service infrastructure. You will eventually make a mistake and forget which timezone your code is running in. Also other services like a SQL Server Database, Service Bus and will use UTC. If you always use DateTime.UtcNow and always expect UTC, you will write fewer bugs and make your code more readable.

A point in time should be localized when presented to the user, not when running through your business logic.

Exceptions

However.... There are times when a localized time or date could be used in code. But you have to identify these cases as they occur, and often in these cases you don't need a full DateTime, it might be a time of day or just a date.

Tools

NodaTime
The only date and time api you will need. Excellent handling of timezones, just keep the database updated....

moment.js
Client javascript library for date and time, support for multiple languages and formatting. I use the ...fromNow(); api all the time.