Grand Theft Auto: Vice City
Purely because it took me months to do, mainly due to being stuck on one of the later missions. I'd also rank it as the hardest GTA i've played as a result (in comparison i flew through San Andreas within 7 days..... GTA IV took a couple of weeks).
Another game i'll mention at this point - the original Leisure Suit Larry.
I used to play this when i was under age, purely for the age verification trivia questions. I loved attempting to answer them, when the majority were questions America would know... yet someone from another country, not have total knowledge of. My parents were ok with me playing it.... as they thought i'd never progress through to the main game. It actually didn't take me long to get through

.
I didn't play much of it until i was in my mid teens, and then the next hurdle was the fact that my original game had became damaged (just after the half way point of the game).
Jump forward a few years later when i acquired the game via the LSL Ultimate Pleasure Collection..... and i finally completed it.