I'll say they are all good cars.. Mercedes is boring but really shines when it comes to AMG and their really sports models. They just started recently appealing a bit to the younger crowd.
BMWs have always appealed to younger crowd. They had the "scream" factor, they were fast and always handled great. Older models had a really aggressive look to them and I liked that about them but with the new design I'm just not digging it looks overdesigned and bulky. The interior of BMWs never impressed me. But they really never went for super tech interior anwyays.. They always went with their cars being the "drivers" car.
Audi always fit somewhere in between. It had offerings for the sporty crowd but most people who buy Audis (including myself) appreciate their amazing craftsmanship of interiors that have always set the bar for the car industry and we got just enough juice in those engines to put a grin on our faces. Audi always made cars that drove excellent and felt super comfortable.
But with recent years, with the appearance of A5 model and after, the R8, the revamped S series, the RS series (RS4, RS5) they really kicked it up a notch. They came close to BMWs with driving mechanics and performance with RS4 and now they have outpaced them with the new supercharged A6 engine and the S6/RS6 series when compared to BMW equivalents.
Now, I will say, that the BMW series 4 looks like BMW is hitting back. That car not only looks insane, it will have most likely amazing performance.
In general I don't really care too much about performance. I'm in my mid 30s and I appreciate the combination of comfort, technology and sportiness even if Audi isn't on top.
So I guess it's a matter of taste. In addition, Audi is the only car manufacturer that has given me so much for being loyal to them that I can't even express it. From not asking questions when my credit was crap, to giving me insane incentives on new models, to actual experience working with people at Audi dealership, it was night and day compared to both BMW and Mercedes dealerships where my experience felt like I should be happy for allowing me to drive their cars and buy from them.
This is why I love Audis and everything about them. The company is awesome, had 26 months of record breaking sales in US and outsold BMWs at one point. The brand is growing and the cars they produce now are absolutely magnificent.