I choose Dave as well.
Adrian is, by far, my favourite guitar player of the 3, his songwriting with IM is second to none but for some weird reason I'm not a fan of his 'solo' work, I find it to generic.
If his instrumental album would be somewhat like Starblind or Isle of Avalon with those long instrumental passages and solos I could change my mind but as I imagine an album from him, it wouldn't sound all that way. He would be my 2 choice.
Regarding Janick, despite being a great songwriter, I think his songs need a voice to become 'complete'. He is not the guy I'd expect an instrumental album, i do not believe his playing would be suitable for a complete instrumental album. It would be cool if he decides to change my mind tho!
And Dave, it's a shame we only have his work with Maiden, I think his playing is so versatile and smooth that anything of music he plays always sound good.
Besides he is the most exotic player of the 3 and I think that would make the difference on a instrumental album.
The picture/sound of a instrumental song by him I have is the passage of The Nomad, it's one of the coolest minutes of their discography. Another example is in The Thin Line Between Love and Hate, I can clearly imagine that song as as instrumental, it think it has so much groove and feeling in it that..specially the solos and the last part of the song. He is clearly my choice on this!
This post is already too long, better stop now!lol