something wide and fast, like a girl I dated for a bit.
maybe a nikon 14-24 f/2.8, or a tamron/sigma 17-50 f/2.8
you could also go with some primes like a 35mm f/1.8 or a 50mm f/1.8
unless the indoor area is very well and evenly lit, or you have some flashes, you want something with a large aperture if you want to expose your pictures properly.
if you are talking about a LARGE indoor area, you could get away with longer lenses, zoom or prime.
there really isnt one "best" lens. the area size and lighting conditions and number of people will determine what lens will work best in that situation, and often times multiple lenses will fill the role just as well.
Didn't say what lenses you do have. As flash can help with slower kit lenses. Upping the light levels of location by turning on more lights. Barring that a more capable f2.8 zoom or fast prime would help.
Events,indoor I mostly go with zoom like my Tamron 17-50 f2.8 as first choice as up's the usability indoors and lower light situations. As many times inconvenient or impossible to feet zoom to compose with a crowd indoors without getting in the way.
For primes would mostly choose 35mm f1.8G over the 50mm f1.8 as can be a tad tight indoors.
And the 35mm gives me a bit more breathing room.
I think it depends on the type of party as well. Often I take photos with aperture set at f/5.6 to f/8 and use flash (even the lens can do f/2.8) just to have a enough DoF to cover the subject(s) I want to be sharp in the photo. Of course, I do swap it with a fast prime once in awhile for a specific shots. (Like head shots / half body shots for a person ).