It really depends on thee factors:
1. How much do you want to spend?
2. How soon do you want it?
3. What sorts of games do you want to use it for?
VR headsets have been in high demand since the release of HL Alyx, and often sold out, so you might be in for a wait.
The Valve index is the best all-rounder currently (apart from lens glare, which is pretty bad), but the most expensive (£925), but it's also on a 3 month waiting list. It has very high resolution, high FOV and can run at up to 144hz. It's an enthusiast's headset which requires decent hardware and fiddling with settings and refresh rates to get the best out of it. The controllers are the most advanced, with full finger tracking, but also have stick reliability issues, and not many games support them natively using all the features. The tracking is the best out of all the current headsets, but requires mounting two lighthouse boxes. Though heavy, the Index is very comfortable.
The Rift S, which is now coming back into stock, is less than half the price (£400), but has some shortcomings (poor built in audio, no IPD adjust (the lenses are fixed so you can't adjust for eye-spacing), Facebook required for social features), but has the best platform - the software generally just works without fiddling. The controllers are the best in class (except for the now retired Rift CV1), being very ergonomic and having partial finger tracking. The inside out tracking works very well indeed, with five cameras, though there are blind spots. The screen is very sharp with virtually no lens glare, but is limited to 80hz. It is a very comfortable headset due to the halo headstrap design.
The Oculus Quest has been constantly sold-out but is now coming back into stock, selling at £400 for the 64gb version. It's the most versatile, being able to run as a standalone headset, with lower quality graphics, or as a PC headset via a USB cable or software. Wireless is a huge game changer for VR and it has to be experienced to be believed. The Quest has an OLED display unlike the Index and Rift S, which makes it less sharp, but with much better colours and contrast, and it has IPD adjust. It is only 72hz, which is very noticable in fast paced games. It uses the same controllers as the Rift S, and the tracking (using 4 cameras) is very good. It also has built in controller-less hand-tracking, which is a bit janky but cool, and some games are now supporting it. The worst aspect of the Quest is the comfort as it's front-heavy. Be prepared to spend money on mods to make it comfortable. It's not as good a PC headset as the Index or Rift S, but works well for the most part.
The HP Reverb G2 will release soon, for £600 and will have the highest resolution of any of the headsets, making it ideal for sims if you have monster hardware. However it is using Windows MR technology and the tracking and controllers are not as good as the Index or Oculus devices.
As for games, all headsets support Steam VR games, but Oculus also have some excellent exclusives. These exclusives can be played on other headsets using add-on software, but there's often a performance hit.