Whimsicott is likely the better option if you wish to have a competitive partner in your team.
It has access to priority Tailwind, which can help your offensive team's middling speed significantly, if they have any. It also has Taunt to prevent the AI from using Double Team, Thunder Wave, or any other annoying status move against you.
Lilligant is equally as awesome if you're planning a powerful Chlorophyll sweeper, but my vote goes to Whimsicott for diverse utility.