See the editorial best maps page for what's editorially most important. This ranking is by download popularity, regardless of editorial value.
What this list reflects
Downloads from the large mod-distribution sites. Visibility matters - old mods have more downloads because they've been around longer. Promotion matters - mods promoted by large creators see an increase.
Popularity doesn't equate to quality. Some of the most-downloaded mods are simply texture swaps. Some of the most ambitious projects have low download numbers.
What kinds of maps tend to lead the popularity charts
Map Editor mods that serve as bases. Interior replacements limited to a single room. Stunt arenas. RP-server-shipped MLOs, when distributed separately. Texture overlays that alter Vice City aesthetics.
Heavy, ambitious projects seldom rise high on popularity charts, as their installation friction is greater and their audience is smaller.