Gens 3 and 5. 3 because Ruby was the first Pokemon game I ever played and my favourite Pokemon was introduced in Hoenn (Treecko), and 5 because of the more convenient changes (Such as being able to use Repels without going to your bag if the last one runs out) and the Unova games honestly taught me an important lesson about seeing things not in black and white, but in grey areas too. Also banger soundtrack and art style! 
