Very good idea to make a contest on the top 5 artists ...
@sarahart, You already know mine but more in detail and in chronological order (by way, I'm waiting your list). I wish you all the success for its realization.

Caravaggio

Cardsharps, 1594-1595
Oil on canvas, 91 × 128 cm
Kimbell Art Museum, US

Trickery and illusion. Stylistically, take as a model a prostitute to paint the Virgin, a lot of courage at that time (he paid for it). He paints real life.

El Greco

Laocoon, 1610-1614
Oil on canvas, 137 x 172 cm.
National Gallery of Art, Washington
First the subject, say the truth but nobody listen and be punished. Stylistically, it's exactly what he said

I paint because the spirits whisper madly inside my head

Lurid color, elongated lines, illogically conceived space are perfect to represent doom. Timeless.

Goya

Duelo a garrotazos, 1820-1823
Mural painting on canvas, 125 x 261 cm
Prado, Madrid

I like a lot of Goya paintings like Los Caprichos but this one tells everything. Why continue to fight to death? The two wrestlers sink into the quicksand but continue the fight. So dark.

Odilon Redon

La Naissance de Vénus, circa 1910 (I think it's 1912, but I want to finish with Duchamp)
Oil on canvas, 143 x 62 cm
MoMa, New-York

For his imagination (you have to see his engravings). And this painting, a little bit of color and love in this list. :-)

Marcel Duchamp

Nude Descending a Staircase (No. 1), 1911
Oil on cardboard on panel, 96 x 60 cm
Philadelphia Museum of Art

Evidently, he defined modern art... Nothing more to say.