Half of these answers are either about Ninjask, or don't even apply to the actual question, so I'll try to look over each Bug/Flying type:
BUTTERFREE:
The first of them all. Bad stats in most aspects, but has a small niche for powdering with Compoundeyes. Fairly unorthodox, compared to Venomoth, but you can use it if you want.
SCYTHER:
A great sweeper that is as straightforward as all sweepers. Just use Technician on it, put some decent STAB and coverage on, and let it rip things apart.
LEDIAN:
Not bad, but has gaping holes in terms of use. It's fairly fast, and makes a decent Special Tank in lower tiers, but use that Special Bulk to set up, not Wall. Not the best choice, but passable.
BEAUTIFLY:
It's an inferior Butterfree, and probably the worst Bug/Flying type there is.
MASQUERAIN:
My favorite Pokemon, and the Bug/Flying type with the best coverage available. Water-type movepool, Bug-type movepool, and a few tutor moves with Quiver Dance/Baton Pass, make this guy a multi-use wonder in the lower tiers!
NINJASK:
Yeah, just read everyone else's opinions.
MOTHIM:
Also pretty mediocre. The mixed Sweeping stats are good, but it's way too slow. Unless you can baton Pass Agility onto it, don't even try using it.
VESPIQUEN:
An ironic Pokemon through and through. Why make it bulky if it can't wall anything because of a bunch of weaknesses? People seem to think it's good, but really, it's pretty average.
YANMEGA:
A formidable Bug/Flying type that has its obvious flaws, but enough assets to keep it strong. A fair movepool paired with healthy stats makes for a unique sweeper with its quirky abilities.
