Hands down Storm and Magneto are more powerful than teams. Really Storm don't even need the X-men, since her powers dwarf their's and she can block telepathy with electric fields.
Day One: Storm and Mags VS X-men team (current or classic, you choose). Not counting herself as part of the team of course.
Day Two: VS The Justice League: Minus Superman and Wonder Woman. The rest I think Storm and Mags would solo. Batman get's a week of prep (not that it will help) Battle happens outside Wayne Manor.