Easily my iPhone 6S.
It's amazing to think how far technology has come. I carry this super powerful personal computer with me every day. I use it from the moment I wake up, to the moment I go to bed, every single day.
My phone has become my primary device for browsing the Internet, posting on forums, listening to music, reading and replying to emails, banking, keeping up to date with news, communicating with friends, social media. It does everything I want it to do, and is easily the most personal piece of tech I own.
I love my iPad Air 2, and would be my second most used current Apple product, but usage of it comes no where near to what my phone usage is. Even sitting on the couch, I'll be doing stuff on the iPad, but my iPhone is still sitting next to it for WhatsApp, controlling my music via AirPlay etc...
My Retina MBP barely gets any usage any more. It used to be my primary source of managing and listening to my music library, but with my 128GB 6S, and great new stereo speakers hooked up to my AppleTV, it's all done through the 6S now. I think the only time I end up using the MacBook is when my iPhone and iPad are charging!