7x7x7 is the largest cube they have official competitions for, I am currently ranked 3rd in the UK for this event so should have some decent knowledge on the performance of the various options out there.
It is important to remember when looking for big cubes that there is always a risk of pieces popping out, when this happens you should obviously know that it is not broken and that it can be fixed, if you are not prepared to fix minor pops then you should not get a big cube.
If you are looking for a budget option then I would first suggest the YJ YuFu V2M, this cube is only £15 but performs pretty well and is the cheapest magnetic 7x7x7, it has very strong magnets but has a feeling somewhat reminiscent of the MGC, you may also want to consider the DianSheng or the YuXin Little Magic, these both have weaker magnets and are faster but lack corner cutting.
For top-range options, I personally use the MoYu AoFu WRM, it is still quite fast but has very good stability, the various lock-ups that can be caused by inaccurate turning are pretty rare, it seems to have an excellent ability to prevent them however I turn, it is also smaller than previous 7x7x7s, it does seem that sizes of big cubes are consistently going down, I do think we are nearing the end of this though. Other options include the YJ MGC and the X-Man Spark, I have used these both in competitions many times, the MGC is faster and slightly cheaper, it is also bigger and blockier but does allow for fast solving. The X-Man Spark is still prefered by a few cubers as it is more stable and has the classic QiYi feel of smoothness, it does suffer from some lock-up issues though so is generally not used by top cubers anymore.
It is possible to average under 2 minutes on any of these cubes given enough time and focused practice but I think most people should either get the YJ MGC or MoYu AoFu WRM.