I have just been through the same selection process and personally went for the Xray XB9.
Thats not to say that I am going to tell you that it is the must have car though, its what suits you best.
What I found from my research was that the Mugen are generally considered to as strong and robust as anything else on the market, but tend to be at the higher end of the pricing scale.
I was told (dont shoot the messenger) that the quality of screws and bearing used in the AE 8.2 are poor, but parts availability is massive and cheap(ish).
The Xray XB9 is getting rave reviews from those that have run them, and I am very impressed with the build of it so far although it is only early days yet. Spares for Xray products are widely available, although those for the XB9 are only just filtering onto the market as it is so new. Price wise they tend to be 'mid division'.
The Agama A8evoFR is the latest revision of their successful buggy and I cant really fault it. This was an extremely close second for me behind the XB9. If you happen to live/race in the south of the country then the trackside support from Nemo Racing is second to none.
The Hong Nor X3 is also fairly new, but the team drivers have been putting in some good results with them. I have held one and had a close look at it, it was probably third in my final selections list.
The Losi 8 appears to have reputation for going very 'sloppy' very quickly (again, dont shoot the messenger!) so personally I sidestepped this one. Also the rumour mill is strongly suggesting a new Losi is imminent before too long.
JQ's THE car had teething problems when it was released (model number ending 377) with the qualitiy of some of the components. There is a new model though (ending 477) which has had these issues resolved and those that drive them seem very happy with their selection. Spares are also freely available and are some of the cheapest on the market.
There are literally dozens more cars available besides these few, but these were the ones I focused on.
Aside from the individual merits of each car you should also consider what people use where you are likely to be racing, what fits your budget best, what your local store stocks (if you have one), what your preferred online stockist sells etc etc.
As I was told though, anyone of those cars will be better than I am. So if I fail then it wont be down to a bad car!
Let us know what you choose.