Rifts is another all-in-one book that I still have after all these years. Patchy as the rules are, I can't wait to run it again. If I could just get the group into it.
It's too bad there wasn't a streamlined D&D 3e. I know some are trying it now with OGL books, but it isn't the same. Most OGL games have some variations that may be great but vary from the core game (by definition), which really cuts into my mastery over the system in unsatisfying ways. The promise of d20 was great, and it achieved modularity for me with many alternate games--my favorites being Omega World and Judge Dredd. Good as they are, they aren't really all-in-one books though.
If not, what do you think should be in a “perfect” all-in-one rpg book?
I don't think a perfect all-in-one book exists, because I don't think a perfect all-in-one game is a practical possibility. Games are, IMHO, at their best when they work with their genre. Since different genres have different needs, no one game can do them all well.
That being said, I don't make perfect the enemy of good - there are a lot of really good single-book games out there, many of which have already been mentioned.
Unless, perhaps, you can edit the rules yourself?
Well, I can always edit rules myself. I feel proper genre-integration takes a bit more than editing - it takes game design of whole subsystems, and is best when matching a genre is done with intent from the ground up. If I wanted to write my one game, I'd just write my own game.
I'm a "right tool for the job" kind of GM - I'd rather have a half dozen games that each do different things an do them well, than one that does them all just kinda-sorta-okay.