As far as I am concerned, the best tandem for most people is the one they have got. We would all love to aspire to the top of the range model, or the one with the best chain set and ancilliaries or the one with the gold plated price tag. (or in the case of Tandems that should be solid gold as none of them are cheap)
I was lucky in that I bought a Tandem that was suitable for the use that I give it which is aggressive off road use. Over the years it has been upgraded to make it even more suitable for its use, but it is still my original Tandem. I don't think I can afford, or find, a better Tandem for me. I have a friend who has one of the cheap tandems. We Don't ride together as although his is Mountain Bike "Type" It will not do the aggressive riding that we do, and I do not like the gentle Trails that he does. We do occasionally do a road ride together, but neither bike is set for that.
Other than the "Wallmart" bikes (And some people are very happy with these) any tandem is adaptable for the use you give it. These things are expensive to buy, to maintain, and upgrade. Decide on the budget you can afford, and buy the best Tandem for you that the Bank will allow you to get.