Guinan, played by Whoopi Goldberg, appears in 29 TNG episodes and two TNG movies. Her character is noted, among other things, for her unique clothing style and especially for her very large hats. The following is a complete annotated gallery of Guinan's outfits.
Guinan can be seen with no less than 31 different costume variations in the 29 TNG episodes she appears in. Four of them are period clothing with appropriately smaller hats: one for the Dixon Hill holoprogram in TNG: "Clues" and three in 19th century San Francisco in "Time's Arrow I/II". The only time Guinan can be seen without a hat on TNG is when she fences with Picard in "I, Borg".
This still leaves 26 different regular outfits. Yet, owing to Guinan changing clothes in several episodes, most outfits can be seen twice during the run of TNG. The only one that appears three times is the red one from "Imaginary Friend", "Time's Arrow II" and "Suspicions". On the other hand, the garish yellow and hence probably most striking costume only shows up in "Hollow Pursuits". Not counting in the period costumes, it is noticeable how the hat color and/or fabric always perfectly matches the gown, the only exception being the very first outfit from "The Child". While usually her dress/hat combinations remain invariable, Guinan can be seen with the same dress but different hats in "The Measure of a Man" and the second time she appears in "The Dauphin". Vice versa, she wears the same hat with different dresses on the following occasions: a purple one in "Q Who" and "Yesterday's Enterprise" and a green one in "The Offspring"/"The Best of Both Worlds I" and "Redemption I". Four of Guinan's extremely wide hats in red, teal, pink and purple are the exact same pattern, just in different colors. Among her tall hats, some seem to share the pattern as well, notably the purple, the blue, the pale green and perhaps the pale red one. Finally, as a general tendency, Guinan's hats become wider as the series progresses.
Curiously, the blue parallel reality outfit from "Yesterday's Enterprise" reappears in the restored timeline in "Family". On another funny side note, in "Rascals" the costumes conveniently shrink as well as Guinan, Picard, Ro and Keiko O'Brien are rejuvenated in the transporter accident. A second, accordingly smaller outfit was made for Isis J. Jones, who played young Guinan.
When Guinan is first seen in "Star Trek Generations", she has just been rescued from the Lakul, after briefly entering the Nexus. When Guinan appears to Picard inside the Nexus later in the movie, she should consequently wears the same outfit, although on a closer look only the hat seems to be identical. On the Enterprise-D in the same movie, she can be seen with the red outfit with the wide hat from TNG for the fourth time. When Picard talks to her in private, Guinan wears a different red dress, and without a hat.
Guinan's so far last appearance is in "Star Trek Nemesis", with a so far unseen costume in toned down colors that are untypical of her.
Many of the screen caps from TrekCore.