Hello. Just want to say I LOVE the game. Definitely captures the best of my favorite WC3 tower defense games. I personally used to play Elemental Defense and Soldier Tower Defense the most.
As far as the challenge... The first suggestions in this thread were good, and I'm not going to repeat them.
What I'd like to see is your top 3 towers ranked based on the number of kills. Total number of potions consumed, total number of items/cards transmuted, total number of creeps killed the entire game, number of attacks per second (all towers, assuming there is always a creep in range) worst tower would be cool, but hard to rate with towers like Ganesha that don't really attack, MOST Assists... For the tower that got the most his before another tower killed the creep, number of towers sold/destroyed.
That's all I can come up with for now! Man, this game has what every TD game on Android is missing. Keep up the great work!
(Survived 299 seconds)

Spirrow,
I love the assist idea. However, depending on your tower build it may mean almost every tower is an assist..

As before stated from Zeek and some of my ideas.
-Total Game Stats:
Wins: X
Loss: x
Total Game Length: xx:xx
Gold Farmed: x
Max Gold: x
Creeps Crushed: x
Best Hit: x
Total Kills: X
Bonus Round: X times
Bonus Survival: x seconds
Total Bonus Round: X seconds
Top tower: towername (calculated with best hit and creep crushed)
-Single Game Stats:
Difficulty: easy/normal/hard
Gold Farmed: x
Max Gold: x
Damage Dealt: X
Creep Crushed: x
Best Hit: x
Total damage: x
Top tower: towername (calculated with best hit and creep crushed)
Time Skipped: X seconds
Sets used: x
Favorite Set: x setname
Bonus Round: x seconds
-Top Towers:
Damage: X tower
Creeps killed: y tower
Best hit: z tower
-Miscellaneous:
Potions acquired: X
Potions drank: x
Items acquired: x
Items transmuted: x
Towers acquired: x
Towers built: x
Towers replaced: x
Towers transmuted: x
-Best Bonus Round:
Wave completed: x
Difficulty: easy/normal/hard
Time survived: x seconds
-Bonus Round:
Wave Completed: x
Easy: X seconds
Normal: X seconds
Hard x seconds
-Challenges:
Challenge Won: X / Y
Mass Challenge won: X / Y
-Personal:
Level: X
Total XP: X
XP to next Level: x
XP gained: x
Favorite Tower: X towerplaced (Y times) (it's not good for the game to tell which is your favorite, in my opinion.)
Best hit: x + towername
Gold Farmed: x
Max Gold: x
Top tower: towername (calculated with best hit and creep crushed)
I think I'm done for now, i'll add something probably. I put same stats multiple times so that X can make a screenshot of a card assuming there will be a card for each section. I provided 8 section, therefore 8 cards should be able to consult. This thing has to be reorganized tough.
I'm very sorry for this long post, I ask for your comprehension. I apologize.
Thanks for your time.

i know this challenge focus on game stats after win/lose,but..
And i have some suggestion/opinion:
-i think you may create ability to "relocate" (i mean moving a tower to other spot with all the stats remain (including current level and so on))
-and if item cards effect can be upgradeable it would be nice. i have a concept for it. in item cards loot,we can't swipe down,right? i think it would be great to add swipe down to "level up/upgrade" certain item.
(every 4 same item card for one level upgrade,i think that's enough)
for example like Wooden Staff original effect is damage plus 5%,after update it become 8% or 10%.
(and to make upgrading ain't easy,the "price" of upgrading will increase)
for example first level need 4 same card,second level need 5,third need 6,fourth need 8,fifth need 10,and so on. you know,that's hard enough to collect so many same cards.

Level: x
Exp to next level: x
Exp gained: x exp (gained in the game)
Ladder rank: x
Ladder bonus round rank: x
Wins: x
Loss: x
Total creeps crushed: (total of kills)
Best hit: x
Bonus round: x seconds
Transmutations: x

Good ideas Manu!
Haven't forgotten this challenge, it just not enough time right now :-(
But I have a good feeling to add this in 0.11 finally!

Add miss chance to tower stats and add another line for custom bonus or allow it to count meads drank in addition to any custom bonus it already has.
Not sure if these necessarily fall within the challenge but just my two suggestions.