Best first track

 Best Clapton, to me, is Detroit Ballroom, Fall '67- Cream were at their peak live. Sadly, this is not an authorized release. I wish Clapton would legitimate it-- it is purportedly a board recording, but there are at least two different versions. I hate getting behind a bootleg, but it is there as a piece of history. 

As to best "first tracks," I dunno. I listen to a range of stuff, from early heavy rock (now often called "proto-metal") to jazz. I have a visitor tomorrow and was thinking about what to play. I have a lot of records, but got out of the habit of a "short list" of regular players for demonstration purposes. 

And, I'm more into jazz these days than hard rock, but I can dig a psych or heavy rock album for sure: Lucifer's Friend (German Philips the album with Uncle Fester); Crimson- Toronto 2016 Live- Starless; Art Lande And Rubisa Patrol– Desert Marauders (killer jazz record that has an edge; well known cheap heat); Chris Whitley- promo version of Living with the Law-- a/k/a National Steel- fewer cuts, sonically nicer. Standard issue Columbia. 

I'm sure I'll play more tomorrow- but part of it is what the guest likes, no? 

+1  Baba O'Riley from Who's Next.

King Crimson "21st Century Schizoid Man"

Beatles "Come Together"

Supremes "Where Did Our Love Go?"

Rolling Stones "Rocks Off"

Bob Dylan "Tangled Up In Blue"

There's a variation on this question involving vinyl releases.  What's the best B-side opening tracks?

Genesis-Selling England by the pound

Rush- Tom Sawyer

Deep Purple-Highway star

Roxy Music-Do the strand

Led Zeppelin-Whole lotta love

Queen-Innuendo

Steely Dan-Do it again

David Bowie-Station to Station

Pink Floyd-Astronomy Domine

Lou Reed-Vicious

King Crimson-Circus.......

....................

But there are many greats, the list is endless.

Selling England by the Pound Oh Yes!- putting it on now had totally forgotten what a cool album

I'm just a lawnmower, you can tell me by the way I walk