The ZPacks Hexamid Twin Tent is the best three-season ultralight backpacking tent we've ever tested. It weighs a mere 13 oz. with bug protection (17 oz. with a waterproof floor), is spacious for two people and protects against horizontally blown rain and high winds. If we were to have one tent for three-season backpacking we would choose the Hexamid. Nothing else comes close to providing this much comfort and protection for so little weight.
However, ordering the Hexamid is not convenient since it is not available from major retailers and only directly from the small manufacturer in Florida (made-to-order, involving long delays in delivery). But, if you can get past the idea of waiting, you won't be disappointed. If you're in search of an ultralight tent immediately, consider the MSR Twin Sisters, which retails for $300.
If you are looking for an ultralight shelter that can also be used for other activities, such as climbing and skiing, we suggest a flat tarp like the Hyperlite Mountain Gear Square Flat Tarp, which is much more adaptable and versatile than the Hexamid's fixed pyramid shape. For winter travel or multi-sport expeditions we recommend the Mountain Laurel Designs SuperMid, which provides bomber storm protection and space for up to four people. All of these shelters and others are compared in our Ultralight Tent Review.