An undisputed, comprehensive best way to learn a language isn't rational. That's like saying this job is the best job in the world. All jobs, by definition, make you some money. Some are much better than others, obviously; however, the 'best job' is only exists in the form 'the best job for you.' It is highly dependent on your goals. The best way to learn a language is as well, unless you're a child. Learning a second language as a child is by far the best way to learn a language, however we are going to assume language learning as a second language learner.
A few important questions to ask yourself when learning a new language are:
Why do I want to learn this language?
What strategy, course, resources, or combination of all am I going to use?
How much time do I have to dedicate to this.
Anyone can learn any language at any time. The only variables are speed and to what level. Anyone can learn a word or two in any language. If that is your goal, and you have 10 minutes to spend one day, you will accomplish it very quickly. If you're goals are bigger than that, which most are, you will need to spend a bit more time on it.
After choosing your language and answer your "why," here are a few steps to help you get started:
I like to start off with some sort of structured program; a class, learning software, course, etc. This is easier because they have already done the work to go through the process of 'how to begin.' Most people begin the same way, there isn't much variation.
Once I get to a decent beginner level, I start reading a lot and trying to listen/watch videos as much as possible. I keep track of all of my 'new words' and grammar points and study them separately.
Once you get to a point where you can consume information (articles, videos online, watch movies with or without subtitles, etc) it gets a lot easier. At this point it's just about keeping up with doing things in that language and learning. It is less 'intentional study' and more learning through acquisition.
After learning a few languages to relative fluency, nobody's definition of fluency is the same, I can have a specialized, personal method that I now utilize. This method, for me anyway, is the only way I have found success; not only in results, but in sticking with language learning. I have nicknamed this the plug and play language system.
When you learn a language, many doors open up in your life. You not only understand a whole new culture, but you understand your own just that much better. It's a beautiful experience, and one everyone should live at least one time in their life.
Get learning a language today! Devote time, study daily, be consistent, and you will speak your language before you know it.