I voted hastily but this is a really tough question. These are both undoubtedly in the top 5 pop songs of the 00s. The violin-perfected Toxic v GaGa's full-throated orgasmic roar in the final chorus. They are so different. I refrain to say which way I voted.
It's so unfair, because I grew tired of Toxic since it is a seven-year old song now. I voted for Bad Romance as I consider that both songs are catchy as hell but Lady Gaga's vocals outshine every Britney track, Bad Romance has a better video and its coreography is genious.