The ultraportable market has evolved significantly since Apple launched the original 2008 Macbook Air. In fact, most laptops today would be considered "ultraportable" only 5 years ago.
Laptops with desktop quad-core processors like the Macbook Pro and Dell XPS 9550 are only 0.6" (1.5cm) thick and weigh 3-4lbs (1.3-1.7kg). Only hardcore gaming or budget bin laptops are considerably larger.
Save HXC gaming laptops, there are three classes of notebooks: cheap ultraportables with slower but reasonable performance and long battery life, ultraportables with powerful performance and battery life, and laptops with desktop CPUs and dedicated GPUs.
Cheap ultraportables ($250-400) include "Chromebooks," Dell Inspiron, Nokia Lumia, Lenovo A10, Asus Transformer, and ThinkPad Yoga. They all feature 8-12+ hours of real use battery life. They often include various gimmicks such as: tablet mode, detachable keyboard, touchscreen, stylus, and various screen types. Though they use slower processors, they're fast enough for 95% users. Many are even capable of low resolution video editing and 3D gaming. One common complaint is their "compact" keyboards.
Ultraportables with next level performance ($450-900) include Apple Macbook 12", Apple Macbook Airs, several Lenovo ThinkPas, Lenovo Yoga Pro, ASUS ZenBook, Microsoft Surface Pro, and Dell XPS 9350. Some have touchscreen or tablet-esque gimmicks. These are for power-users who utilize video editing and advanced gaming. Ultraportable laptops feature high resolution screens with more accurate colors, solid chassises, and larger keyboards. Despite an Intel Core processor (instead of an i5 or i7) the Macbook 12" features was never slow, simultaneously running Hearthstone, a web browser, and other background applications without a hiccup.
Desktop replacement notebooks ($800-3000) include Apple Macbook Pro, Dell XPS 9550, many Lenovo Thinkpads, Razer Blade, Microsoft Surface Book, and HP Spectre. These laptops can spend their whole life on your desk connected to 3 external monitors; delivering the power of a fast desktop computer. While powerful, they only weigh a couple pounds more than the aforementioned ultra-books, and only slightly thicker. Advantages includes a beautiful screen, delightfully tactile keyboard, and a solid state-of-the-art chassis.
I am a big fan of buying used laptops off sites like eBay or craigslist, especially for models 6-12+ months old. You can save 30-75%. PayPal buyer protection is often unneeded: I have traded many laptops without trouble.
For the best laptop in terms of power and value, SF Tech Beat officially recommends the used Macbook Air 13", which sells used for around $400. The dual core i5 in an indestructible 3lb body is a killer combo. I would also keep an eye out for the successor to Dell's XPS 9550.