Flyers can be a bit of a bug bear in 40k; you need a 6+ to hit them if you haven't got skyfire you're going to have a problem. Add there's not a lot of weapons which have access to skyfire - some armies have hardly any access to skyfire at all i.e Eldar and Tau.
I did an article sometime ago on how to deal with flyers: Tactica: how to deal with flyers which gave info on what each army has in their arsenal to deal with flyers. The question is, what is the ultimate anti flyer weapon?
Before getting onto what I think is the ultimate flyer weapon, I'll throw in a very good contender and that's the quad gun which is available with an aegis defence line or a bastion. The quad gun is a gun emplacement which is T7 2 wounds and has a 3+ armour save. Dakka dakka wise it can fire four S7 twin-linked shots and it also has the skyfire and intercept rule. Best bit is every army has access to this, however it costs 100 points overall including the aegis.
I'll throw my hat into the ring and say the ultimate anti flyer weapon is the tesla destructor; a Necron piece of kit which fires bolts of electricity.
If you don't know about the tesla already then let me tell you; it is S7 AP- heavy 4 twin-linked. At the moment that doesn't sound awesome, but Necron tesla weapons have a funky rule which when you roll a 6+ you get an additional two auto hits! So that single hit on a flyer suddenly turns into three hits!
If that's not good enough all units within 6" (friend or foe) can have the tesla arc off onto them on a 6+ and take D6 S5 hits.
The tesla destructor comes mounted on an Annihilation Barge which is a steal at 90 points and also a Night Scythe, which only costs a 100 points and is also a flyer!
What do you think is the ultimate anti flyer weapon in 40k?