Richard Reti (1889--1929) composed a number of great endgame studies. Here are two of my favorite pawn endgame studies.

The following study is perhaps his most famous, beautiful in its simplicity.

[Event "Kagan's Neueste Schachnachrichten"]
[Site ""]
[Date "1921.??.??"]
[Round ""]
[White "Reti, Richard"]
[Black "Study"]
[Result "1/2-1/2"]
[FEN "7K/8/k1P5/7p/8/8/8/8 w - - 0 1"]

1.Kg7 h4
    (1...Kb6 2.Kf6 h4 3.Ke5 h3 4.Kd6 h2 5.c7 =)
2.Kf6 Kb6
    (2...h3 3.Ke6 =)
3.Ke5 1/2-1/2

The study below was dedicated to Julius Breyer, and is based on coordinate squares theory.

[Event "Berliner Tageblatt"]
[Site ""]
[Date "1923.??.??"]
[Round ""]
[White "Reti, Richard"]
[Black "Study"]
[Result "1-0"]
[FEN "4k3/8/3p4/p6K/P5P1/8/8/8 w - - 0 1"]

1.Kg5!
    (1.Kg6 Ke7 2.Kf5
        (2.g5 d5 3.Kh7 d4 4.g6 d3 5.g7 d2 6.g8=Q d1=Q =)
    2...Kf7 3.Ke4 Kg6 4.Kd5 Kg5 5.Kxd6 Kxg4 6.Kc5 Kf5 7.Kb5 Ke6
    8.Kxa5 Kd7 9.Kb6 Kc8 =)
1...Kf7
    (1...Ke7 2.Kg6! {wins as below.})
2.Kf5 Ke7 3.Kg6 {(zugzwang)} 3...Ke6 
    (3...d5 4.Kf5 Kd6 5.g5 +-)
4.g5 1-0