"Best" is certainly subjective -- there are lots of high quality balsamics out there that can range from $30 for a 200 ml bottle to a couple of hundred of dollars. A great way to explore is to go to the olive oil bar at Pusateri's and they will let you taste pretty much any of the balsamic vinegars they carry. A really good option is their house-branded 12-year old balsamic. Good value for good quality. Thick, rich and sweet. But there a lots of other options if you're willing to pay even more.
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i totally agree with the above. you don't even necessarily have to buy from them but at least you'll get an idea of the flavour of each brand. taste in balsamic is completely subjective and therefore you need to actually try it. generally though, if they don't let you try the stuff, be wary.
personally, i don't like it aged to oblivion, especially if the stuff they start with isn't great. reminds me of ribena, actually.