For groups, answer is easy -- Primus EtaPower 2.1L -- weighs 8.8oz with pot, lid and handle (at least, from what I can tell looking at my gear list -- can't remember for sure if the removable handle is included in that number, but I think it is since the pot would be hard to use without it). 2.1L is plenty of room to cook for 3-4 people and the fins on the bottle help fuel efficiency. I've used it extensively with isobutane setups, but I've also been successful cooking in it with a Zelph GoTo alcohol stove.
For solo or 2P trips we take a GSI Halulite Minimalist (0.6L). It's not the lightest piece of gear, but it's very functional: 6.1 oz includes the pot, lid, neoprene sleeve and silicone "gripper" (instead of a handle). It works well heating water for two people and the design of the lid and the silicone sleeve mean that it doubles as an insulated mug for hot beverages -- that functionality is worth the weight.
In both cases, prices are cheap. The Primus EtaPower 2.1L can be picked up for ~$20 after discounts, coupons, etc. (check coupons and Sierra Trading Post right now); the GSI Minimalist is ~$22 on Amazon or other places.
I'm sure there is better gear out there, but I haven't found anything that competes with either option at the ~$20 price point.