I think it's called Vice City Stories. It's one where you have Gangs and you need to take over buildings. But then at any time, your buildings can be attacked. I remember taking every building on the map and still being attacked. Was annoying. Never completed a GTA in my life, lol.
It's all about making vehicles on the tallest building and then driving them off the edge lol. If you spawn a hovercraft, WITHOUT the flying car cheat, it'll still start flying if you drive it off the top, lol.
Now that I think about it, I've never beaten any of the GTA's... There is always a mission I can't complete. Plus, I get distracted driving around the cities or fooling around with the cheats. Sometimes I try to see how many policeman I can kill before they stop me... The key is to find a good spot when helicopters can't get to you
ive completed 3, vice city, san andreas and 4. havent played any of the "stories" titles though. of the ones i have played i dont know that i can really pick a hardest. im not sure if its because i found them to be very similar, or if because i simply dont remember, lol.