Hi xara,
You appear to have been well provided for in terms of ready-made lists compiled by others (nod to Jo & paulb), personal favourites (nod to belM & emanuela), personal hates (nod to shanks) and also more general advice along the lines of "follow your feelings, Luke" (Obi tsuwm Kenobi)!

I think any list of supposed all-time greats has to tell you almost as much about the person (or people) who compiled the list as about the quality of the books recommended. Yes, there are generally acknowledged classics, but there's no point in reading them just because they're acknowledged classics. Some of them are deadly boring, or may not be appropriate to you and your world at this time. They may not resonate.

I like tsuwm's comment about being eclectic. I'd take it a bit further and say - IMHO - the best books are those that move you. If you can't avoid some subjectivity then try embracing it.
So here's a loose list of books that have moved me. I'm not going to stop and think about it, or it'll change..

Iain Banks: The Crow Road and/or Espedair Street

Ray Bradbury: almost anything really, but in particular I'm thinking of a short story where a young woman or girl is walking home through a dark wood and a murderer is meant to be on the prowl. Can any Ayleurs place this??

Dr Seuss Brilliant stuff! Particularly recommend The Lorax, though I do like Green Eggs and Ham. And I would read it in a box and I would read it with a... (That's enough of that. Ed)

Hilda Boswell's Treasury of Poetry
Every child should receive a free copy. Excellent poems with wonderful pictures.

Harry Harrison: Star Smashers of the Galaxy Rangers and/or A Transatlantic Tunnel, Hurrah!
The former is very, very funny. The latter is more clever, but is also very amusing.

Ira Levin: A Kiss Before Dying and/or This Perfect Day
AKBD is probably worth reading even if you've seen the film. It's use of an "invisible" first person (the murderer) - something you can only do in a book - is fantastic and enthralling.

There you go. That's fairly eclectic!