What is the most efficient technique used to gain altitude with a fighter or any other plane? I have seen many ways of doing it, the simplest being a standard climb by accelerating forward at a slightly upward angle.
   
I have also used a series of [Immelmann's][1] to achieve the desired altitude.

Are there any other techniques that use a different game mechanic to achieve a certain altitude the fastest?

[1]:http://wiki.warthunder.com/index.php?title=Combat#Immelmann "War Thunder Encyclopedia"