What does a hard game mean to you? One of my points of view: it means a game not in one's native language(s), requiring at least 2 dozen ticks. I think the learning aspect will increase the fun of the challenge. In this light, I'd nominate the following two games: "Greater Germany in 1941" and "Was gibt's zu essen?" Both created by RonaldDerGrosse.
1) http://www.purposegames.com/game/greater-germany-in-1941-quiz
2) http://www.purposegames.com/game/744

Also, any of those games in Russian created by Weissmann.

I think a tournament comprised with various multilingual games would be fun. Also, I'd recommend at least 13 games in the tournament.

Some of mine might work as well:

British, Ottoman, Roman or Greek:

http://www.purposegames.com/game/british-ottoman-roman-or-greek-quiz/info

Steam Locomotive:

http://www.purposegames.com/game/a1e88ca3/info

Disputed Territories:

http://www.purposegames.com/game/765

Primes and Factors:

http://www.purposegames.com/game/2834/info

Missing Map:World Turned Upside-Down:

http://www.purposegames.com/game/375bc08a/info

Alphabet Soup III : My Brain Hurts!

http://www.purposegames.com/game/alphabet-soup-iii-my-brain-hurts-quiz/info

And some pairs that might go for the easy and hard:

The Eye {by Nilas}:

[With my own] The Human Eye (Advanced):

http://www.purposegames.com/game/1756/info

and two of mine on one subject:

Neuron:

http://www.purposegames.com/game/1875/info

and

Neuron (Advanced version):

http://www.purposegames.com/game/2047/info

I also think that RDG may have some games that he should nominate...

I think there are any number of cracking games on the website.

My personal favourites include:
- the RDG classic, 'Shakespearian Tragedies';
- the JGEO game, 'Asian bodies of water (updated)' - perhaps my all-time fave, because of its combination of level of difficulty and aesthetic appeal;
- sergiaragon's 'Capitals Quiz'.

Then there are some others that are almost impossible to play, but all the more fun for that. These include the mind-bending 'The Counties of Great Britain' and any of Tickman's US letter/town series. Re the latter, I'm still plucking up courage to play 'M' and 'N'!

Another game that should be in is the no-clues version of the periodic table. As someone who has failed every single science exam in his life, this would cause me sleepless nights - but, hey, it would be (perverse) fun trying!

The hardest game that I have personally played here, and recieved 100% is the entire table of periodic elements by number:

http://www.purposegames.com/game/382/info

that game is crazy hard!

Actually you could have a tournament called the "58 DAL- Mations" all games by Dal, himself, which are guaranteed to give you an Egyptian, Persian, Levant, Antiquity, Archeological challenge! Thanks all for the game plugs on Shakespeare etc. As the bard would say, "To be or not to be!" "Out, out Blue spot!"

I would really laugh, erm, love to see this game in a tournament!!

Last 5 Heads of State - Africa 2008
http://www.purposegames.com/game/last-5-heads-of-state-africa-2008-quiz

Who gets the 100% wins the tournament *LOL*

i just made a game that i challenge is impossible.... it wouldn't let me put any more dots down, well maybe someday I myself will get 100%. Its "all" the Islands from the Lake I live on.

http://www.purposegames.com/game/9f7b152f/info

There appears to be a glitch in the Last 5 Heads of State that stops you from completing the game - I've now tried this three times and on each occasion the game has frozen one or two dots from the end.

Veejay:

You got a laugh out of me on that one. So OK, it was maniacal laughter, but a good hearty laugh all the same. I didn't even read the URL before I clicked on it.