I'm incredibly biased towards anything Gen 4, but I honestly think the graphics are gorgeous. Smooth, colourful, detailed... really easy on the eyes.
Gen 5 had nice graphics as well, but like Nah said, it's rather pixelated. I liked how the trainers were taller and less squat.
At first I had my reservations about X/Y when it was announced, but once I started playing it that all changed for the better. It's graphics are pretty, with some great trainer designs, though I did not find the chibi look appealing!
And Gen 7 still looks great. It feels so clean; even walking is a joy for how smooth it is. One thing that bothered me is your trainer's unnerving smile—it was either that or a slightly shocked expression, haha.
I love the old games for looking so Pokémon-y though. They're certainly more nostalgic!