Starships by miles. From the corny rap verses, the overly autotuned vocals, and that awful dance breakdown after each chorus makes the song easily the worst. I feel like Super Bass is more of Nicki's signature song though than Starships, which is a better song.
From best to worst, I'd say:
Umbrella>Toxic>Crazy In Love>Firework>Party In the USA>Poker Face>Love Story>Tik Tok>Problem>>>>>>Starships.
Ranking: Toxic > Beautiful (X) > Umbrella > Love Story > Crazy in Love > Poker Face > Super Bass > Tik Tok/Problem > Party in the USA > Star Ships > Firework
Not Starships being Nicki's signature song. Sure, the song was an undeniable smash but it was forgotten by the end of the year. Her real signature songs are Super Bass, Anaconda, Monster, Itty Bitty Piggy, and Feeling Myself (now )