The cheap chinese replacement E39 specific head units are 'most' likely all made in some factory in deepest China somewhere and then the 'brands' you see put their software/skin on etc and then flog them on ebay mostly. Dynavin is currently the best of this bunch as they follow a distribution channel route that's much better. This means dedicated exclusive dealers/importers in each territory, proper sales and after care backup support, better guarantees/warranty claims if needed etc. None of the other cheap chinese made units offer this. There's one or two that have been flogging cheap electronics on ebay for years, Eonon is one name you may see, however they openly put up on forums that they'll illegally misrepresent the price in order to avoid import and local taxes/duties etc, so if they're willing to break the laws of countries like the UK and USA etc, I'd doubt they'd be anything but helpful if you need support.
In short, your term of 'best' is open to interpretation, personally, I'd never put cheap, generic low quality car audio electronics into my car and will always prefer the premium brands as they use high grade everything for their products. If you are after excellent sound quality, a properly made product, peace of mind in terms of a quality product with aftercare guarantee/warranty, get a premium branded head unit at the expense at the loss of 20% oem 'looks' in your dash. If you just want a replacement head unit with none of the above and sort of looks oem, choose one of the cheap chinese units at the risk of having 'issues' with it etc.