Nintendo Switch is my best. It has already cemented itself as one of my favorite platforms of all time — I really enjoy most of its exclusive games and find that its hybrid form factor really fits well into my lifestyle. I'm authorized to play games at my work during off time so portable mode opens up some really great binge gaming opportunities that most other playforms just can not replicate at this level of ease.
Worst would be my first gaming laptop purchase, which was an early 2017 MSI GS65. I'm typically a big premium ultrabook user, so moving to a midrange gaming laptop turned into kind of a nightmare of losing a lot of creature comforts in just using my portable computer. Gaming performance was impressive for something so thin and light, but fan noise was more immediate and constant than even the worst consoles and there were no audio solutions that could drown it out, the screen was dim and made a lot of viewing experiences tough, the keyboard had mushy response so normal typing and gaming both felt whack, my trackpad was erratic and lacked fluid response any time I tried to use it for casual non gaming tasks, and even when not gaming the battery life lasted like 2 hours. I sold it and went with a non-gaming laptop for a year and had a much better user experience even though I couldn't game for a while, and just a couple of weeks ago I got a mid-2019 Razer Blade 15 on sale and it has so far felt like a 500% more premium mixed-use portable computer, even though it cost significantly more (even though I got it on sale, to boot). Battery life and typing experience don't quite measure up to my typical ultrabook experiences, but neither are awful and using it outside of gaming legit feels like using a pre-2015 MacBook, so I love it even when I'm not gaming. My old MSI absolutely sucked to me after my honeymoon phase over its gaming performance (vs its form factor) faded.