I don't think there is a "best" to be honest. They all have advantages & drawbacks.
Ally - I'd say this is now the basic no-frills Windows handhelds of choice. Following its price cut, it's around £150 cheaper than the Legion Go for similar performance. You just forgo some of the Legion's more interesting features - the larger screen, the detachable controllers, the track pad.
Ally X - this is probably as close to a "best" device as you can get. But it's £800
At the end of the day, it's just a regular Ally with 32GB RAM, a larger SSD, larger battery, and a minor redesign. At the heart, it's still the same Z1 Extreme processor powering things. It needs a hefty price cut.
Legion Go - largely covered above. Offers interesting features, but it's more expensive than some better performing rivals. Personally, I'd love to see the Legion get the Ally X treatment.
Antec Core HS - great value at £550, especially if you're quick & get the accessories bundle with it. One of the top handhelds for performance & it has a built in keyboard. However, the screen is only ~6", or about the same size as a smartphone's screen.
MSI Claw - plagued with issues. Best avoided.
Steam Deck - the most "console-like" experience of all these handhelds, with a great screen & best in class battery life. However, the latest Windows handhelds are
a lot more powerful.
Also, while the Linux OS is great for getting maximum performance out of the hardware, there's a growing gap between what the Deck can handle and what the latest Windows handhelds can do. Running on Linux also has some drawbacks. Many common Windows tools simply don't have compatible Linux counterparts. Mods can be more difficult (or in some cases, impossible) to install. And if you want to play Game Pass titles, you have to either dual boot Windows, or stream them from the Cloud.
I'm happy with my Steam Deck. But I'm reluctant to say it's the best. The reality is that the hardware simply cannot handle some 2023 & 2024 releases. And this is a problem which is likely to get worse in the coming years. It's also disappointing to have to jump through hoops to play Game Pass titles.