Never said it should be. However, it failed both as a "masterful, compelling narrative" and as a fun crime romp since each chapter ended with "lol u guys suck so hard u so dumb" rather than feeling as though you, you know, actually pulled off a crime. If I wanted the first (well "masterful" per the low bar of video games), I'd play Mafia II. If I wanted a fun crime romp that felt fulfilling, I'd play Saints Row the Third. GTAV failed to hit either of those marks or even fit in a comfortable intersection.
I suppose it has the "open world" thing going on but I'm not especially interesting in playing video golf or video yoga or video rollercoaster.
I don't think GTA5 had the best narrative, characters, and whatnot of the GTA series - but it might be the best overall game to me. A lot of this game's elements, it ticks the boxes of what players want out of GTA. Big sandbox to do stuff in? Yup, major check. A good lead? Heck, you have three different leads here - just find one that speaks best to you. Michael's another version of Tommy Vecetti, but he has issues with his family; Franklin's another CJ and has his own issues; and Trevor's just every GTA player that wants to do insane, crazy, and off-the-wall stuff in the game-world...but Trevor's insane madness happens in missions too.
I dunno, but for me...best narrative and story probably goes to GTA4 (base). Niko, Roman, and Dimitri - man, was all that stuff good. But, for most people - they didn't like that it really didn't have the fun factor that other GTA's had; GTA4 base was meant to be more realistic. The "fun" stuff felt like they got bolted onto each GTA4 DLC more and more - as fans wanted more "fun". TLATD was more fun than GTA4 base; and Tony went hog-wild with crazy over-the-top missions with just the first mission alone, setting the tone...and continuing on that route for the rest of the game.
GTA5 took the "kitchen sink" approach and felt like a greatest hits of many GTA's. It also had a lot of the complaints of "long missions" w/ autosaves in the midst of them - so if and when you failed, you didn't have to go back and start over from scratch. My Gosh, seemed like each GTA had a crazy mission that would drive you nuts until you got it right - i.e. VC with the RC Mission; SA with the train mission & one of the Chinatown missions. Pretty much, GTA4 solved this - but GTA5 kept on w/ this.
About Mafia - Mafia series, both Mafia 1 and 2, are great; especially w/ the story & character stuff.
I wish I could finish up Mafia III, as that game is also great at story, presentation, and narrative - but eh, it's got issues. A bunch of them. Mafia 3 ain't the best running game; it has too much repetitive main and side missions; and a lot of the side stuff can get in the way of doing the main stuff. Mafia 3, for better and worst, got turned into an open-world UbiSoft-style Mafia title - and Mafia series was at it best, being linear and story-focused.