You really CAN'T not love Gen 5 in terms of playability. They've just added so much stuff (Triple Battles, Rotation Battles, Join Avenue, The World Tournament, PokeStar Studios, etc) that it's easily the most enjoyable and has the most versatility.
After that, I'll go Gen 2. Why can't more Generations replicate the multi-region experience? Sixteen gyms and two major champions to best? Heck yeah.
Gen 3 and Gen 4 comes down to personal preference, and I just prefer 4. I loved Sinnoh, and I thought every aspect of it (starters, villain team, champion, rival) were superior to Ruby/Sapphire.
Gen 1 was the game-changer and great for nostalgia, but I feel every generation since has vastly improved on its formula.