Youtube told me to watch this dude's videos about top 20 drum intros...and then I watched top 20 keyboard and guitar intros.
https://www.youtube.com/watch?v=LpY_9qY-j1g
So.
What's a great intro? This dude said the single chord intro to A Hard Day's Night was #1, I call BS. I think it's Voodoo Chile by Hendrix, which didn't even make honorable mention. We agreed on the best drum intro, and #2 on the drum intro took me by surprise but I totally agree.
Keyboard intro? Hm. I didn't like what this guy came down with, but there were some solid choices in the list. I'd say either Foreplay/Long Time or Light My Fire. Baba O'Reilly might be the best though.