How should I order unknown wines in a tasting
Light first, see e.g. this list for ordering them by varietals, if you have old world wine you need to decode areas to probable varietals :-) 

So for reds that would make a list like this.


Pinot Noir
Sangiovese
Tempranillo
Grenache
Zinfandel
Merlot
Shiraz / Syrah
Cabernet Sauvignon