Certainly in my top 5
To start, there are not a lot of games out there with the wild west theme, so right there was a big change. The campaign was very intense and never got old. It was a good length too, not too long or too short. Aside from the main story, there are so many side missions and activities in the game to add even more hours onto the game. Five finger fillet, duels, horse racing, horse shoes, poker, liars dice, bounty hunting.... the list goes on. I would also like to mention that some of the side missions even gave me the creeps with what story lay behind it. You can also become a town hero, or the most known crook, where you would have a bounty on your head. There are so many interesting details and things you can do in this game, it gets you addicted. You can even just explore the vast world with free time.
For the multiplayer, this was probably what sucked me in the most. All the competitive game types, the free roam, the leveling system, almost everything about it I loved. There was so much to it I didn't really care there was no train or lasso online. I think the lasso would have gotten annoying with real players anyway. I purchased all of the DLC for this game, and I loved every bit of it. They all gave you even more game types and other perks. Especially with Undead Nightmare, which even gave you an extra 6 hour campaign. Rockstar switched up the theme with Undead Nightmare too. It would have been cool if you could swim though, or like, steal rafts and fight with your friends, but it's cool in my book.
(I really enjoyed the euphoria physics engine. It's fun seeing them fly around xD)
Rockstar is an amazing company and has never let me down yet! If you don't mind a few bugs here and there, and are looking for a great, different kind of shooter, this is your game!