I am British and have lived in many countries around the world. I now live in a small town on the Mediterranean coast of Spain: https://en.wikipedia.org/wiki/Moraira (which I call my little piece of paradise).

Moraira is an affluent area where many Europeans like to holiday, but there are only a handful of hotels due to planning restrictions. This leaves rental properties highly sought after and 3/4 bedroom villas (houses) costing EUR 300K-400K are often rented to tourists for over EUR 1,000 / week during the summer.

Be VERY careful when buying. There are countless TV programmes showing foolish Brits who took shortcuts when buying in Spain, which have cost many of them dearly.

Right now it's a buyer's market, but I would suggest only dealing with companies that come with first-hand recommendations.

I live right on the coast in what is one of Spain's hidden jewels. Go north, south along the coast or west a few miles and you will get even more for your money.

After having invested in many properties in Central and South America, my best advice to start is to set your personal investment goal first. What wort of property are you looking for? Residential house? Condo? Apartment? Commercial Property? Do you have a bank account in Spain already? LOCAL real estate professional? Attorney to help guide you? Have you been to Spain already? Most of the very best deals and professionals are not found on the internet but by visiting the local areas in Spain you are considering and asking around. There is no substitute for this boots on the ground due diligence. You can make money or lose money in any area of Spain, you must first know your investment goals and criteria and how this will fit into your current lifestyle.
This is something I would love to talk to you about if available.

I don't know about Best Investment in Spain.
But If you are looking for Best RE investment period, it doesn't need to be in Spain, I would do RE investment within the Commonwealth of Nations. If you want to expend in Asia, Japan and Hong Kong would be the best options.
I'm curious, why specifically in Spain?