My most recent installation - I purchased Teyes brand stereo, - this is the best one I've touched so far.
Mazda
They have several tiers of products: low, mid, top. I have personally chosen mid tier for better quality display - CC2pro.
For both gen Mazda 5, there are two options: go for 7" generic "2 din" or for 9" with a specific frame. Second-gen - I would definitely recommend going for 9 Inch, - this looks great. First gen - probably, 9 inch as well, although, to me it looks a little bit oversized.
Support is great, they also have plenty of additional devices that integrate almost seamlessly: TPMS sensors, all kinds of rear- front- and dash cameras.
I absolutely love my wide-angle reverse camera, I find it incredibly helpful. They also have 360 surround camera kits that integrate with their systems.
The biggest downside would be that their fan/modders community in mostly in Russian on 4pda.