Sustainable Bike Choice

Well can of worms.  So Steel requires much less energy to produce than aluminium as a raw material but very little material is raw and often includes a high percentage of recycled content.  Recycled steel and Aluminium have a very silmilar energy cost.  Steel would then be the obvious choice for total energy outlay although i think it could be argued that getting the most out of the Aluminium (as so much energy has been outlayed to produce)could also be argued for if you can guarantee the recycled content.  I did some research at work for a customer on sustainability of steel and i found a letter for a UK steel producer saying in the UK 75% of all steel is recycled.  

I'd advise getting a really nice used steel frame - maybe a Bob Jackson, a Brian Rourke, Saffron, or something like that - and putting a modern groupset on it. LFGSS has a whole thread on it:
https://www.lfgss.com/conversations/212733/

I've found my Rourke to be unbelievably smooth and comfortable even though I can't fit more than 25s (on wide rims) on it. Indeed, I reckon the current fad for very wide tyres is only because of the need for very stiff uncomfortable forks to cope with disk brakes...

I wanted to suggest why not Ti, but found this article instead.

https://grist.org/living/what-is-the-greenest-type-of-bike-frame/

Buy secondhand and buy Shimano's new Linkglide group for longevity and serviceability. Oh and invest in a decent drivetrain lube which means your drivetrain lasts longer. Personally I hate the waste associated with poor maintenance - cable outers, inners, chains, cranksets, and cassettes are very hard to recycle so the longer you can keep them clean and running well the better.

I'm very happy with the stuff in even very lowly Shimano groups- I have just reconditioned my daughter's bike for the other daughter. It's a Halfords Carrera with Tourney, and with everything oiled and adjusted it's perfect with very smooth shifting. I expect Campag and SRAM have equally loyal fans. It's also aluminium and a cheap 'racing bike', and it's the first aluminium road bike I've ridden. They're supposed to be a hard ride on rough Lancashire roads, but I couldn't really tell any difference from my Reynolds steel frames.

Not sure why you would pick SRAM or Campag over Shimano?  I had a bike with Campag a few years ago, and yes it was nice, but getting parts I found harder than Shimano. 

The non-drive side crank arm came off at the end of one ride, the bits had worked loose and I didn't see it happening.  I was less than a mile from home, thankfully.  I saved the crank arm, but the washers and a few other smaller parts ran all over the place.  I remember it took me over a fortnight to get it back on the road, whereas I reckon if it had been Shimano then a few days max.

How Green Is Cycling? - https://www.bikeradar.com/features/long-reads/cycling-environmental-impact/

Personally, I wouldn't worry too much about it. The above article reckons producing a bicycle takes 96 Kg of CO2, that's the same as driving a few hundred miles.

.....a Speedwagen, on mech Chorus - or as Peter has suggested below, get an old bike (er, classic, surely?) and do it up. I got into cycling in the 90s and now have a fine collection of bikes I couldn't afford then - much to the annoyance of Mrs Hayes.  Latest project: Jan Ullrich's Pinarello Prince from the 1999 Vuelta! 

I think my most sustainable bike is my Brompton as that is used in lieu of tube, bus and car journeys. 

Hope have repeatadly replaced the cables on my lights, only recently charged for it.  One of the reasons I went for their disk brakes, that and apparently they continue to support their old stuff, with parts availability, long after others seem to.

Chris King headset, proper quality and the excellent seals mean that they will last longer than any other, plus parts should always be there.

One advantage of my Rohloff is the lack of a need to constantly replace worn cassettes and jockey wheels, and the chain seems to last a long time too. Month and a half of an initially filthy commute, and it's only just hit 0.25%.  I was waiting on a replacement frame last winter, but the shortest quality 8 speed chain only lasted 2 and a half weeks for the old getting to work bike.

One answer would be to buy a second-hand bike - that's surely got to be far more sustainable and gets around the issues with new bike supply chains.

However, I suspect that isn't the answer you want.