I got 100% in Vice City.

The hardest things were hidden packages, "The Driver" race, and the firetruck missions.

I did Ambulance first go =)

I didn't use a guide to find the hidden packages directly, but i did print out a map so when i found one, i'd find out where i was on the map and mark it off so i don't go searching the entire city for the last package.

I got all the hidden packages in San Andreas, but not without an FAQ. I got down to just two pictures left in San Fierro and I got about 70 of the tags in Los Santos before using it, but the shells and the horseshoes were too hard to even start bothering without a guide.

About the pigeons in GTAIV: You do not need a guide for them. I got sent some junkmail (in-game that is), which has a link to a site that shows the locations of all the hidden objects in the game. I'm almost certain you get sent it for completing the story, since it turned up right after I did that. I've marked all the pigeons on the map that comes with the game - no need for a guide/faq!

bump

I got 98 in GTA3, I couldn't find the last 2.

I heard a false rumor that you unlock Caster City, if you find all 100 hidden packages.
This made me search for hours straight.

I found all of them with no guide in GTA III.  Never bothered in any of the others.

I had to use the guide for all three PS2 games to get them. (The real bithch was SA where you had the Horseshoes, the Clams, the Graphitii, the Photos.) There is just no way in hell someone could get all of them with out a guide book. Anyone who claims they did better have the 1000 hour game save to back it up.

 Wish I'd have known that before I shelled out $15 does it show the stunt jumb as well?. I expected more collectibles than appear to be in the game though. 

@rasone77
Yes, all the stunt jumps, all the random characters, all the cars to steal, all the weapons, health and armour, and one or two other things. I agree, the game has removed too many good things that SA had and not replaced them with things of equal quality, or indeed replaced them at all. I don't think the game is as good as SA, but that's for another discussion.

Btw, you should use gamefaqs.com instead of paying for guides, I've rarely found that it doesn't have the answers I'm looking for.

I did in GTA:VC, but I never bothered in GTA:SA. Mostly because there's about 500 Hidden Package equivalents in SA.