Please define "best”. Cheapest, fastest, best views, or anything else?

But you could start with having a look at Seat61:

https://www.seat61.com/Portugal.htm

Hi ​@rvdborgt thanks for replying. I think best for me is good connections and not too many train changes. 

Thanks for that url, very helpfull!

Do you just want to get to Portugal quickly, or do you want to make a stopover? You’re going to have to spend a night or two anyway, so the departure ti

I would look at getting to Spain first. Some options:

• Eurostar to Paris, cross Paris, then TGV to Barcelona, AVE from Barcelona to Madrid (3 other operators don’t accept Interrail)
• Intercity to Brussels, TGV to Lyon (or somewhere else in Southern France), TGV or AVE to Barcelona or Madrid
• Eurostar to Paris, cross Paris, then TGV to Hendaye, 

The northern route via Hendaye has more changes, and less frequent connections. But might be more useful if you plan to go to Porto.

If you’re going to southern Portugal, consider train to Huelva in SW Spain, then bus. Much quicker than the slow 3-train route from Madrid.