Probably in the top 3 of best ARPGs I've ever played. Often considered the predecessor to Grim Dawn this game uses the (if done correctly) amazing class system of choosing two separate masteries to create your class. There is a lot I could say about this game but I'm going to start off with the negatives first.
-The Cons-
1. This is not your modern ARPG
Before anyone jumps on me and calls me out for not being a "true" fan of the genre or anything, I actually PREFER this. I LOVE that this game is not your modern, 1 button pressing, legendary loot at level 10, inconsequential builds ARPG. This to me is NOT a negative but it must be stated because if you are NOT interested in putting at least 10+ hours into this game you probably will not see what I see, and 10 hours is kinda low to be honest. I will get more into this when I talk about the classes/builds but just know you probably won't feel like a "strong" or "special" character for a much longer time then you'd be used to coming from modern ARPGs. Couple this with the hard bosses (Typhon WILL stop your run the first time you playthrough, that's fine, enjoy it, start a new character if you'd like, don't stress), rare loot drops, most abilities have cooldowns unless meant to be your autoattack, and you're going to be drinking potions like your life depended on it because it normally does.
I know this is a negative point but I must say from my perspective I'd really give this a try still. The bosses are meant to be slow burn events, loot is meant to be exciting when you get it (and many people have beaten the game with yellow, or uncommon, only gear), ability management USED to be a part of ARPGs and I'd wager the genre has gotten worse for moving away from this, and while drinking endless potions can be annoying 'E' and 'R" are your best friends.
2. Quality of life
While MOST of the quality of life features I can overlook the two I cannot defend is the game "remembering" what difficulty you last played on (many times I accidentally play on Normal with my Epic+ characters). Speaking of I'd actually commit ARPG blasphemy and say that APRGs SHOULD move away from the 3 difficulty system, while I understand the point of it and don't MIND the idea of running older zones with harder mobs, I just think forcing the player to "start over" 3 times is quite archaic. This is admittedly very minor but it is very annoying, however this game also ESSENTIALLY has no stash and this is a genuine concern.
The stash system in this game is SO extremely minimal that if you have even 2+ characters and would like to keep gear for either future runs or just for any sort of collecting you will run out of space probably by Act 3. There are work arounds TQ Vault is a great resource after I ignored it for years I now use it all the time. This is definitely a genuine complaint with the game that I can't defend or argue against.
-The Pros-
1. Classes/Builds
Arguably every ARPG lives and dies by its classes/class system. I could probably write a 100 page thesis on why classes are important for ARPGs but this isn't about that, it's about Titan Quest, just know that I cannot understate how important it is to have a compelling class system and Titan Quest definitely does.
So this game has a total of 11 masteries, with DLC, you as the player at level 2 choose your first mastery that can be leveled up to 40 with new actives and passives after leveling up your mastery. Once your character has leveled up to 8 you choose your 2nd mastery which then creates your class. While your class doesn't generally affect any stats (masteries do 'class' doesn't) most people understand what you're trying to do when you mention your class (for example if I say I'm playing a Conqueror veterans understand I'm probably building auto attack heavy character).
All this to say, just surface level, there's a lot of playstyles, but then I'd even argue each class has multiple niches to them. Want to play an archer with a lightning orb pet? You can. want to play a trap placing dual wielder? You can. Want to play a fire mage with wolves? You can. (disclaimer you CAN not saying all these builds can push Eternal Embers Legendary, I simply don't know)
Then we have.
2. Itemization
The gear/class system of this game makes SO MANY interesting combinations of fun things to do. I remember the moment I fell in love with the itemization of this game was the moment I found a shield (can't remember the name, sorry veterans) that had a chance to on block create a huge temporary pet ON a character that used shields and pets. There's many examples of this but especially in Epic+ there's many items that will seemingly validate your builds you've been working on.
Again, loot is rare and specific loot is even more rare, but there are so many items out there that validate dozens of different builds to at least make it TO Legendary, probably further.
5. Everything else
From the setting, to the enemy diversity, to the boss battles, to the heroic monsters, to the massive content available. This game truly is special and as I said before is top 3 ARPG of all time with a MAJOR argument it could be number 1.
P.S. I hope TQ 2 is half as good as this.