You may want OpenGL 2 for compatibility with some older hardware, but if you are going to use raw OpenGL, if you stick with OpenGL 3 (Core functionality), it will be much less confusing and you will learn modern OpenGL a lot faster. My suggestion is to learn OpenGL 3 (ignore anything that is lower than this) and stick with it. As a bonus, you will find transition into OpenGL ES (iOS, Android) easier, should you decide later to pursue it.
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