Charles Rennie Mackintosh (1868 - 1928) believed everyone that came to the table deserved a throne. His high backed dining chairs not only radiated royalty but seemed to praise formality in the everyday. Mackintosh came early to the Modernism game and his designs embodied the movement well: Rejecting tradition and even burlesquing it but never without a subtle reverence for it. (Always best to give a nod to the giants that came before.)
More of a niche than a chair, this Mackintosh design shows more than a little of an Asian influence (above). Clearly, it's a design that still resonates as demonstrated in the Philippe Starck chairs (below).
In many of their chairs, contemporaries Frank Lloyd Wright and Mackintosh shared a similar vision. Two of Wright's master works below: The Barrel (below, first) and the Robie House chair (second below).
Before Danish architect Arne Jacobsen bombed the furniture biz in 1952 with his first major success, the innovative Ant (below left) chair, he contemplated the future of Modernist design from the seat of his favorite Eames plywood chair. Soon after, he'd follow with Number 7 in 1955 (below right) to even greater effect. The ne plus ultra of Danish furniture design, Number 7 would go on to sell over 5 million units.
Jacobsen continued to produce iconic pieces throughout the 50s and 60s: Most notably, the Egg (below), Swan (second below), and Drop (third below) chairs.
As students, Eero Saarinen and Charles Eames would collaborate on a collection of furniture that would win first prize in a 1940 exhibition at MoMA "Organic Designs in Home Furnishings." By 1946 Saarinen and Eames would follow their muses seperately: Eames with Herman Miller and Saarinen with Knoll. For the Finnish born Saarinen, this fertile muse would lead him to some of the most renowned chairs ever designed. This included the Ball chair from 1946;
the well furnished playboy's pad staple, the 1968 Bubble chair;
the 1948 Womb chair;
and the chair made famous on the starship Enterprise, the Tulip from 1956.
The Jorge Ferrari-Hardoy designed Butterfly chair would come to rule bourgeois homes of taste in the 60s, even though by this time his 1938 design was practically middle-aged. 
Here, the Butterfly elegantly coerces Sophia Loren's knees into the sun.
In 1955 sculptor Harry Bertoia won "Designer of the Year" for what was to become the iconic Diamond chair.
Another chair with considerable aesthetic stamina is the Xavier Pouchard designed Tolix French Café chair from 1934.
The 1006 Emeco Navy Side chair is probably one the world's best known. Founded in 1944 to create a chair for the U.S. navy worthy of a torpedo strike (company founder Wilton Dinges tested the chair by tossing it out of a six-story window), Emeco fabricates the chair in 77 step that is still guided and crafted by hand. (Apparently the curtains may be closing on the traditional process soon.) Despite their sturdiness the chairs are lightweight, the result of being made from corrosion-resistant aluminum. Interior designers would later discover the chair for themselves and pluck them from maritime obscurity. And the rest is aesthetic history.
More recently, Emeco has been producing chairs from recycled materials. This, their plastic version, is called the 111 for the fact that it's made out of 111 recycled Coke bottles. It also comes in a range of colors beyond red.
African-American designer Nathaniel Alexander was first to patent the folding chair (1911). (The idea of the folding chair goes back to the Egpytians.) Though, his version still had a ways to go as it didn't include folding legs, he certainly was a forerunner in what might be the most pervasive chair in modern life. If success in design has anything to do with commonality and use then surely Alexander is one of the giants of the chair.
Even more awesome chair shizznit to come in The Definitive Guide: Part 3; Part 3.
