SQL Server 2008 R2 upgrade to 2017 I need to upgrade and migrate SQL Server, SSRS, SSIS to the newest OS version and SQL Server version.

Server OS is currently 2008 R2 with SQL Server 2008 R2 Ent. installed.

My original plan was to do in place upgrades and then migrate to new server OS. It sounds like SQL Server 2017 is not supported on 2008 R2, so I'm not sure whether or not this is an option. 

What is the best migration/upgrade path for server OS upgrade and SQL Server version upgrade?

Windows Server 2008 R2 and SQL Server 2008 R2 Ent -> Windows Server 2019 and SQL Server 2017 Ent.
As always it depends... 

Whats the complexity of your instance?
How many SSIS packages / SSRS reports do you need to move along?
How much time do you have to perform this upgrade?
Whats your maintenance windows within which the service can be inaccessible?
And many many more...

Before you start looking for any kind of answers I would recommend to create an inventory of what exactly do you have.
Very helpful thing is to add source control over your entire DB server. With that in hand you can "mask it a bit" and then share with the community - much higher chances for good answers.