I know you probably think i'm an uber car audio dork, but seriously, this poll is like posting one saying 'Which brand that makes all car parts (tyres, turbos, windows, seats, gearboxes, etc) is the best?'
It doesnt work that way. Alpine,Clarion and Pioneer all make great headunits, but then other brands like Digital Designs, JL Audio, Adire, Kicker, Resonant Engineering, etc, dont make headunits but make speakers and subs which wipe the floor with the headunit-brand speakers...
As with anything, your best bet is to buy the best brand for whatever particular job you are doing. So if you buy a pair of speakers, buy from a brand that makes good speakers. It doesnt need to be the same brand as the headunit...
The only people who prefer you sticking to one brand are the relevant company's marketing department... If you want a good system, dont decide to stick to one brand, because you'll pay more for less sound