use another format ? have you edited the videos ?
It's mostly YouTube at fault. By that I mean YT uses a very low bitrate when converting a 1080p upload. It only uses 1/3 to 1/4 the bitrate it uses for 1440p uploads, despite 1440p being only about 78% more pixels. It doesn't matter how big a bitrate you compress with, beyond 12000 Mb/s it will just go to waste. That is why they recommend 12000 bitrate for 1080p uploads in their guidelines.
As for the other person's video, it looks to me like they were maybe using an ENB mod that sharpens the foreground and blurs the background. However it's also at night, and GTA V always looks better at night than day.
After many frustrations experimenting with pretty high compression settings on my ShadowPlay captures, I finally tried the resize trick. It used to be you could use a 3rd party tool to resize to 2048x1152, and it would trick YT into using a higher bitrate, but they caught on to that and now it will only use higher rate for 1440 uploads.
I was worried resizing to a whopping 78% larger res, vs 7% larger with 1152p, would just make the video worse. After all, you cannot add in the extra pixels, because it's captured at 1080p. However the results were MUCH better, and it's because YT uses 3 to 4 times the bitrate for 1440p.
YT recommends 30 Mb/s bitrate for 1440p uploads, so just keep your average bitrate at that. I start with 50 Mb/s in Shadowplay and use Avidemux to compress to 30 Mb/s and resize to 1440p. Vegas Pro also has the option to rezsize to larger video resolution. Skip to the 6:00 mark in the below video for how to do that.
[video="[URL]https://www.youtube.com/watch?v=4Mivp_9pYy4[/URL]"][/video]
These are videos I've uploaded at 1080p and after resizing to 1440p. As you can see the difference is quite clear. There's no more blurring and pixelating when moving fast on or over detailed ground with foliage. Not even the smoke or dust blurs or pixelates. You'll need to watch them fullscreen to really see how much difference there is. The 1440p one looks good even full screen on my 32" TV.