There are many excellent 5x5x5 speedcubes currently available and the differences in performance are lower than for larger cubes and even layered cubes, there are a few worth considering:

High-end options:

DaYan NeZha: The DaYan NeZha is currently my main 5x5x5, it is pretty fast but suffers from some stability issues, it feels like a slightly improved version of the MGC with a somewhat similar feeling to it. It is the sort of cube you probably want to try before committing to it, it does have an issue with caps coming off sometimes so that is somewhere to be aware of. It also comes with another version with slightly stronger magnets.

AoChuang WRM: The Aochuang WRM was my main before the Nezha was released, it is not quite as fast as the NeZha but is a bit more stable, it is more expensive than the NeZha at £32 and is not used by that many fast speedcubers, people who do edge pairing on the M slice are more likely to prefer it.

QiYi Valk 5: The Valk 5 is a more stable option, it is not as fast but is still used by many top speedcubers, as of writing this I still have some available but they are no longer being produced so the time will probably come when they are no longer available. 

YJ MGC: At only £17, the MGC is more of a budget option but I do think it belongs in the high-end options, especially as many top cubers use it. It is not as stable as the valk or the Aochuang but does still perform very well.

Budget options:

QiYi MS: The QiYi MS is a £14 budget 5x5x5, it is larger than most modern 5x5s and has quite a blocky feel, however, it is quite stable and turns reasonably fast, most people would probably prefer spending the extra £3 on the MGC.

Meilong M: The Meilong M is a magnetic version of the original Meilong, it has a different colour scheme to the standard meilong series, it is smaller than most other options at only 60mm, when set up well it has the potential to perform well but it is safer to go for another option.

YJ ZhiLong: the YJ ZhiLong is smaller than most 5x5x5s at only 58mm, this means it could be preferred by cubers with smaller hands, it is somewhat similar to the MGC in design but does have differences.

Ultra budget options: 

QiZheng S2: The QiZheng S2 is only £8 but is not magnetic, it is larger than the meilong and may be better but is probably a bit more stable.

Meilong: The Meilong is another very cheap budget option, it still has reasonable corner-cutting but without magnets it is not going to perform as well.