Features shouldn't even count, Christina fans really use that advantage because without Christinas tacked on and pointless feature on that Maroon 5 song in 2011 they'd have to truly come to grips with how irrelevant shes been musically for coming close to a decade. Say Something did well but otherwise that shouldn't be seen as an affirmation of anything. At least Kesha managed a #2 hit with her last failure of an album, Timber shouldn't really count either though but at least her feature on that was actually the driving force behind the success of the track, Christina did literally nothing for that Maroon 5 song. It would've been as much of a hit without her.
Well, from this list Ke$ha if we're talking solo #1s. Her life isn't really in a state for preparing a smash pop album whereas everyone else doesn't have excuses.
Taylor - Nah, she'll have plenty more.
Katy - Yeah, she'll probably have a few more.
Kesha - Probably not.
Rihanna - Yes, she's got 13, she has more on the way.
Xtina - No.
Gaga - I'm not 100% sure, but I personally think she will. Everyone is acting like ARTPOP str8 murdered her career, she'll come back.
Beyoncé - Maybe, maybe not. I'm not sure.
The two obvious answers are Britney and Xtina. Britney's #1 singles were all because of first week sales and hence they debuted at #1(which will never happen again for her+ that remix thing also won't happen again). Xtina can't even get a hit let alone a #1. Gaga is also another one who won't see another #1. Ke$ha is a toss up depending on this lawsuit and Beyonce doesn't care enough nor does she get enough radio play on pop formats to get it. The rest could.
and I'm being honest she can get another one if she's willing too.