Gen III had so many quirky and interesting pokemon designs that make it feel like it's own distinct environment...I don't think the other gens had quite accomplished this regional unity.
Gen V is a bit polarizing despite it's greatness. Most of my favorites are from gen V, but it did have some misses, it's a numbers game, but generally, I look forward to gen V most because I love so many pokemon it allows for me to enjoy each run through without reusing the same pokemon. Example: Picking Lilligant, Serperior, Sawsbuck, Leavanny, or Whimsicott as my grass type is painstaking!
Gen VI is also polarizing. There are fewer unique pokemon, but of the few new introduced, there is high quality! I do love variation over anything else though, so no matter how much I love these designs there is a limitation. Nonetheless, even restricting myself to get 6 pokemon, I still have trouble choosing pokemon.
Gen I is awesome! Simplest of all the designs, but they are super-effective. The drawback? Well, the simplicity? I do have less options choosing teams due to being less enthused to use pokemon I am not particularly fond of. For instance, grass is my favorite type, and I only "like" Venusaur and Vileplume. I feel like with so many pokemon, there should be more favorites. Overall, there is cohesion and lack of ugly overcomplicated designs.
Gen IV has great original pokemon and many poor legendaries + new evolutions of previous lines. I love Roserade and Froslass, but tangrowth, lickilicklycilk, magmortar, electrivire, Magnezone, Probopass, and even Mamoswine a bit tend to be bulky/bloated and non-innovative let downs. It just cannot be ignored in a gen with so few non legendary pokemon. These are the worst legendary mascots, regigas, heatran, and the lake trio rank at the bottom of all legendary designs. However, the starters, vespiquen, luxray, roserade, froslass, lopunny, mismagius, garchomp, gastrodon, drifblim, among other (mostly new lines) of pokemon make up for some of the less attractive choices.
Gen II had great addition to gen I lines like blissey, scizor, crobat, kingdra ect...however, the new pokemon introduced were few and often not interesting, with the exception of Houndoom, Ampharos, Meganium, Piloswine, and Lanturn. It's very limiting making a team of pokemon unique to this region. But there are at least a few new pokemon lines that I enjoy..,this does feel like an extention pokedex from Kanto.