Titan Quest: Legendary Edition just dropped on the App Store, and it's maybe the third or fourth time the game has been re-released on mobile. This one promises to be the "best" in the sense that it includes all DLC for free and a few bug fixes. There are also promises of things like controller support coming in the future.
To be clear, this new edition of the game doesn't feature any visual upgrades, and if you have the older Titan Quest HD, you aren't suddenly missing out on anything. Both games have the same DLC available now, with the only difference being that the DLC in HD is offered as an in-app purchase as opposed to being free from the jump.
I took some video of both Legendary Edition and HD to confirm that there is virtually no difference between the two games in terms of experience or visuals, which you can check out above. The only real decision-making you have to do is around whether you want to go all-in and have the DLCs or buy them piecemeal. If you already own Titan Quest HD, I'm not sure there's good motivation to pick up Legendary Edition, at least not for now.
As far as action rpgs on mobile go, Titan Quest is a pretty solid one, though it certainly shows its age these days. Originally a PC game from 2006, this mythological take on Diablo can look a little crude and visually bland. Rest assured, it does the whole loot lust loop just fine. Just know it's probably not among the best at it at this point.
If the comparison video isn't enough or you don't want to scrub through it, just know that I also took comparison screenshots of both versions of the game in preparation for this piece and I honestly can't remember which one is from which game. See below: