I think the title says it all really, but still, although the answer to some at first may be 'The latest, it has more stuff' but this isn't always the case, as now weather teams are dominant, ridiculously so. This isn't necessarily a bad thing, but many people prefer variation.
So as a brief overview of competitive battling in each generation:
Generation 1: Psychic Dominance, Agiliwrap Dragonites being irritating, and Hyper Beam having better (borderline broken) mechanics. The generation home to the notorious RBY Mewtwo, which was nigh on uncounterable.
Generation 2: Stall. I don't think there's much else to say. Battles almost always last over 100 turns, usually over 300. Creation of the SkarmBliss core, which provided a wall for both Physical and Special attacks. A lot of mind games took place in this generation
Generation 3: Perfect counter to SkarmBliss was made in Blaziken. Stall became less dominant, and a more balanced form of play was made. The introduction of abilities meant that many new tactics were found. Jirachi was introduced, being the first to effectively make use of ParaFlinch tactics. Choice Items were introduced.
Generation 4: Introduction of U-Turn, much faster gameplay. I haven't played much Gen 4, so I'm not entirely sure what it was like.
Edit: Courtesy of Spriter SR|XL, you now have a competent description:
Also, leads were very common, with Pokemon like Metagross, Azelf, Aerodactyl, Machamp, Roserade and Swampert being the best leads. Fire/Water/Grass cores were common, and the matches were much faster than 3rd Gen matches.
Generation 5: All about power. Base 100 speed is no longer a rarity, Stall is much less prominent than ever, and weather teams are pretty much all that is seen.
My personal favourite is 2, I enjoy the longer matches and the mindgames, as well as the fact every Pokémon will always have a set stat, as no natures and no Max total EV cap meant they'd all have maxed out in every stat.
So, what are your favourites?