Story
You play as Niko, Johnny, or Luis in Liberty City. The game's plot is different depending on who you play (obviously), but each part has one thing in common. Stolen diamonds. As Niko, you arrive in Liberty City to visit your cousin, Roman, to look for a better life. However, all is not as it seems and Roman has gotten himself into a lot of trouble. You save him time and time again, get yourself involved with stolen diamonds, and work for others to get money.
As Johnny, you play as a biker belonging to The Lost MC. You start with bringing Billy back from jail and engaging in gang warfare. Eventually, you (again) get mixed up with diamonds and a weird turn of events from your gang.
As Luis, you're involved with the diamonds basically from the start. You are partnered with Tony Prince, a man who runs a series of nightclubs across town. Most of the story is about helping Tony with his problems (drugs, debts) and the Mafia.

Storyline length
The original game, with Niko, took me quite a while to finish. I think it would take around 14 hours to finish if you are focused on just the storyline missions, as I was. The other two parts are significantly shorter. TBoGT took me around 5-6 hours to fully finish, while TLaD is the shortest of the three, having around 20 missions and taking about 3 hours to finish. The game is replayable and has tons to do outside of the story.

AI
Not bad! Their aim, unlike in the previous GTA games, is pretty accurate from time to time. Sure, they miss shots at times, but it feels like the word "cover" actually means something in this game, for once. Their driving could be better, especially that of the taxi drivers. I once got into a taxi, set my destination, and stepped away from the computer for 30 minutes. I came back, and the guy was stuck HORIZONTALLY in a tunnel. How that even happened is beyond me.
My only real complaint is that at some points in missions, the AI is not supposed to die until a certain point (especially when driving/fleeing from you) and it's incredibly difficult to tell when exactly you can shoot to kill.

Experience
Great. Tons of nostalgia came back playing through the trilogy again. The stories are amazing and full of action (though the original one with Niko drags on for a while), memorable characters, fantastic (and fun!) weapons, a huge variety of vehicles, and a large map. Driving feels similar to Vice City's driving, except with even more weight added to your car. The suspension of cars feels "exaggerated", if that makes sense, so driving feels very smooth and just fun overall. I'm also a big fan of the ragdolls in this game.
Wasn't a big fan of the 100% requirements added to missions in TBoGT. Shooting doesn't feel very good in this game, also, which is a huge part of GTA. It feels almost awkward to aim with most weapons - especially snipers.
Depending on which character you play, it almost feels wrong to drive certain vehicles or use certain weapons. As Niko, it's free reign. As Johnny, it feels wrong to drive a car or use a sniper rifle - even though you have free reign. As Luis, it feels weird to drive "normal" cars or use shotguns that don't have explosive shells. This highlights the brilliantly well-done character design.

Graphics
What you see in the screenshots is what you get. I'm here to talk about the atmosphere. GTA IV, the original, seems to have this dull sort of filter over it, which perfectly shows Niko's feelings and the story in general. TBoGT has a more colourful feel to it. Taxis look unnaturally yellow, for example. This highlights the wealth of Tony and Luis and, in my opinion, refers to the story missions feeling like they were made by someone doing cocaine.
TLaD has a very noisy/gritty layer over it. I'm not sure how best to describe it, so it's better if you see it yourself. It fully suits the campaign and the relationship Johnny has with certain characters like Bill and Ashley.
Other than that, explosions look fantastic, and I love how vehicles can get almost fully smashed up and morphed into a cube when you crash.

Sounds
The voice acting was fantastic, and I loved the effort put in by everyone with the accents. The Jamaicans felt like Jamaicans, and Niko felt like a Serb. As someone living in Ireland, the McRearys could've had a better Irish accent, but it wasn't too bad (American influence, I guess).
Every GTA game I play has fantastic radio stations, however, GTA IV's radio stations were pretty... mid. I often found myself scrolling through them instead of picking one and sticking to it. I did, however, enjoy most of Vladivostok, Liberty Rock, K109, Tuff Gong, and Electro-Choc.

Overall
Truly a fantastic game and trilogy. This is what true GTA is, not GTA V or GTA: SA. I'm not saying the others are bad, but if I were to pick a game that defines what GTA is, it would be this.

Specs
It took me an unnecessarily long time to get the game to run well enough to play. I had to mess around with game files, the settings, and compatibility modes for hours before I found out how to play without stuttering or this weird zooming-in during cutscenes.
Otherwise, it ran great the whole way through at 60FPS at all times.

Intel Core i5-10400F @2.9GHz
NVIDIA GeForce GTX 1660
16GB RAM
Windows 10