Not just where (eg: SQLite...) but also how (libs, best specific practices)?
You probably don't want to store sensitive data in shared preferences. Instead you might want to look into a plugin like this: https://pub.dartlang.org/packages/flutter_secure_storage
import 'package:flutter_secure_storage/flutter_secure_storage.dart';
// Create storage
final storage = new FlutterSecureStorage();
// Write value
await storage.write(key: 'jwt', value: token);