My favorite is the Dreadnaught because of the Hive Architecture and the insane amount of loot opportunities (chests that require keys).
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I love Venus for its golden age intellectualist-style architecture, as well as the landscape itself being lush and green; but my most favorite aspect is the calming rain. The constant rainfall super-saturating the planet as I slowly walk through the seemingly infinite marshes. The drops of rain beading of my visor as volcanoes erupt in the distance... all this while Halo 3 ODST soundtrack plays in the background - simply beautiful.