Best brand for pre built gaming pc



None as far as I can recommend. The usual problems with pre-built PCs is that they'll lure you in with a good CPU and a good GPU with 16Gb of RAM (which turns out is at 2133Mhz). But then everything else is of poor quality. Motherboard, case, case fans, and most importantly the PSU.

I would highly suggest you try and build one yourself. You'll also be able to get something much better for gaming with optimized parts.

PCPartPicker part list / Price breakdown by merchant

CPU: AMD - Ryzen 5 1500X 3.5 GHz Quad-Core Processor (£111.59 @ Aria PC)
Motherboard: MSI - B450-A PRO ATX AM4 Motherboard (£79.98 @ Ebuyer)
Memory: Team - Vulcan 16 GB (2 x 8 GB) DDR4-3000 Memory (£108.65 @ Overclockers.co.uk)
Storage: Kingston - A400 240 GB 2.5" Solid State Drive (£28.79 @ Aria PC)
Storage: Seagate - BarraCuda 1 TB 3.5" 7200RPM Internal Hard Drive (£31.79 @ Aria PC)
Video Card: XFX - Radeon RX 570 8 GB Video Card (£161.73 @ Amazon UK)
Case: Corsair - SPEC-05 ATX Mid Tower Case (£46.99 @ AWD-IT)
Power Supply: Cooler Master - MasterWatt 550 W 80+ Bronze Certified Semi-Modular ATX Power Supply (£54.43 @ CCL Computers)
Total: £623.95
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-11-24 19:37 GMT+0000

Something like this would be pretty good for gaming. You could drop to 8Gb of RAM for now as most games won't need that much and get a better CPU like the Ryzen 5 2600 as you can always just add another 8Gb in the future.
 
MSI and Asus do offer OEM's pre-builts but are usually overpriced, and MSI and Asus are mostly part oriented (PC parts like mainboards, GPU's etc.) A good Gaming OEM PC company that I can think of would be something like Digital Storm. They are not as cheap as Cyperpower, Ibuy, etc, but tend use higher tier parts like better power supplies, motherboards, and memory.