Hm. Is validly related to forum games, though, since it's more or less a discussion of what you like in them and what makes them good.
As for me, I don't know that I could point to one and say it's the absolute best. Several of the absolute best have been as follows, though:
Arcanum Octet II. [Link] An RTD with skill-leveling and creative magic use and combination. Consequently, every turn felt like Christmas, as you waited to see how much Fire Magic experience you'd gain or whether carving that rune onto your armor made you invincible or cut too deeply and shredded it into not armor anymore. It had a fairly long run, was several years ago, and was pretty detailed; there were several armor slots, for instance, fractional damage resistance was possible, and damage was split into Piercing, Slashing, and Bludgeoning.
Not coincidentally, several of my own forum games (like
Staggered Magi RTD) and one of my other favorite forum games (
Westlands RTD) were heavily based on it.
Potash Maker Adventure and Potash Maker Adventure II. [Original] [II] An illustrated forum game with that RTD-ish You Die feel and skill leveling. You play as a dwarf with a completely useless profession (Potash Maker, Soaper, Crutch Walker, etc) and are somehow thrown into battle, conspiracy, drama, and more. Mostly as serious as dwarven peasants are likely to get, but occasionally veered into (usually audience-inspired) madness. The pictures added immensely to the feel, the possibility of getting less useless and better equipped was nice but didn't usually work out (in a good way), and the GM was very good at making things interesting. It's probably a year or two old, and it wasn't massively complex, but did feature an area hit system with different damage multipliers on each part from armor and innate value (ie hitting the head was more damaging than the leg).
Elves of Amanareli. [Link] An outright famous suggestion game featuring a band of elves trying to make their way past hostile wilderness with a trading caravan. Superb artwork and brilliant storytelling help make it a true masterpiece, along with the usual insanity that is to be expected from Bay12's suggestions and a creative GM.
Probably the only community/suggestion game I'd consider for a list of the best things ever (on this forum, at least;
Prequel is a stunning example of something similar elsewhere).
EDIT: Wait that's a lie,
You are minifig was awesome.
The Empress shall rise again!Obviously, not very complex, at least as far as we were aware. It did feature precise stockpile recordings, but I don't think that included food eaten or other such nonsense, just the supplies we lost to frogmen or whatever. I have no idea when it was last active, actually, but it did die and then shamble back at least once, possibly more.
I could go on, but I suppose I'll stop for now. Other candidates include Knight Otu and Vanigo's civ building stategy games, several Dwarf Fortress: The Forum Games (especially Shade-o's, but Hastur's and Neyvn's were pretty good as well), and probably some stuff I've forgotten. All of which died at least in part due to overcomplexity, incidentally.