Hmm, that's a tough one...
Prowler - Catchy tune, but I hate the lyrics and I think it's better live with Bruce.
The Ides of March - I haven't listened to this much (can't even remember how it goes - all I remember is it's an instrumental), so no comment. It's the only opener which is also an instrumental, I think.
Invaders - One of my early Maiden favourites, great song and wonderful introduction to Bruce's vocals (Invaaaders!). Lyrics are good, tune is great, no complaints about this one.
Where Eagles Dare - Great introduction to Nicko's drumming skills, plus great song. Again, no complaints, though there are many better album openers.
Aces High - Great song and great introduction to a great album - Need I say more?
Caught Somewhere In Time - I like it, but no comments really.
Moonchild - Err... I actually don't remember this song very well either, so I won't comment.
Tailgunner - Ok song, no complaints really but it's not a great song either. One of the weakest album openers.
Be Quick or Be Dead - See Tailgunner; I don't like Bruce's vocals here that much, and it just doesn't cut it with me. Perhaps my least favourite opener.
Sign of the Cross - This is a great epic that I have only recently begun to appreciate. I prefer Blaze's voice to Bruce's on Rock in Rio. However, I think it shouldn't really be an album opener.
Futureal - Fast and furious, Blaze's speed of vocals is great. An excellent song on an otherwise sub-par (in my opinion) album. Excellent as an opener, though I'd like to hear Bruce try it too (though I think Blaze would still be better in this case).
The Wicker Man, Wildest Dreams, and Different World are also some of my favourites, but I can't really describe them in much detail because they are quite similar IMO.
My favourite is either the Wicker Man, Wildest Dreams, Different World, Aces High, Invaders, or perhaps Sign of the Cross. Sign of the Cross is perhaps musically the best, but it doesn't serve well as an album opener. If I had to choose, I would say Wildest Dreams right now, but that changes pretty often.