I will never be able to answer these questions in a way that would hold up over time. I love them all, in their own way, and I've recently noticed again how I cycle through the shows in phases. Last year, around the time of STXI's release, I wouldn't have let any non-TOS Trek into my DVD player.
Now, I have stocked up my collection considerably and like to watch episodes across all series, but I find myself gravitating towards TNG and VGR mostly. Somehow I have no desire to watch DS9 whereas I'm discovering a lot of gems in VGR that I had previously overlooked, apparently. Which is why I've given my vote to VGR this time, if only so that it gets some support. Right now, it's my favorite, but that'll change again. Historically, my #1 remains TOS.