Both the songs and their videos are immensely overrated and not particularly good, either in general or relative to each artist's other songs and videos, but "Toxic"'s video in particular always bored me. "Bad Romance"'s video is too tryhard and far less clever and artful than people give it credit for, but at least it's a little interesting, if only because it's a complete cluster**** of way too many ideas crammed into one video.
So I'll go with "Bad Romance." But both have better videos.
Toxic is unknown. Bad Romance is the forgotten background music of every heterosexual who got drunk in the club 5 years ago and ****ed some random chick. Both are only remembered by those who had it playing on their iTunes while they posted a Craigslist ad.