there's no definitive answer to this and I believe there is a lot of rubbish talked about oil some people have very strong opinions, ie oil A is fantastic while oil B is utter sh%te etc etc, the truth is all the branded oils are much of a muchness, I'm not saying all oils are the same they aren't, cheap non branded oils should be avoided likewise car oil, but any difference between premium brand 4 stroke motorcycle oils will be negligible.
Aprilia recommend 15w50 semi synth so most use 15w50 semi synth, but depending on your climate/season 10w40 is also an option, IMHO the best option for a UK based bike is stick to what Aprilia recommends 15w50 semi, check out Opie oils for an idea of what's available and what it costs, which brand is up to you (I generally use Motul but if Silkolene was cheaper I'd buy that) use any decent brand and you won't go far wrong.