I asked a similar question as well not so long time ago when I got back into birding. The answer i got was "Handbook of the Birds of the World". Sounded easy to me at first, but I got a quite a shock when I realised what this actually is
LOL. Yeah, I can see how that would be a surprise.
If you're just talking about identification, the best you can do is get the best field guide for each individual region. But just a warning, most will pale in comparison with the Collins guide, some extremely so.
I'm really curious now - what would be the least number of field guides one would need to have coverage of almost every species, say 90%? I must admit, I'm not versed in the field guide options for Africa and Asia. But here's a first crack at it:
Western Palearctic - Collins
North America - Sibley. You may even be able to omit this region, since the widespread and migratory species would mostly have coverage in other guides. But there's probably enough exclusive species to necessitate inclusion.
Central America - I think you could get most of the species by sandwiching the region with Howell's Mexico guide in the north, and then the Panama one in the south
South America - the 2 volume guide to the birds of Northern South America would cover a lot. But if I remember correctly, it doesn't include most of Peru, so you would almost have to add in the latest Peruvian guide. And maybe the new guide for Brazil as well. Not sure how many species that would leave out
Sub-Sahara Africa - ??
Australia - pick the best one of the several available
Asia - no idea how to split it up.