Just done some quick calculations and it seems that actually the best tower is the ice tower, due to its range it has an advantage over the nature tower of 2 levels at the fist corner (level 52 before it gets over run).
Tower rangexdamage=total damage
Fire 120 x 7021 = 824,520
Water 300 x 7335 = 2,200,500
Nature 215 x 9000 = 1,935,000
The water tower has an advantage of over 250,000 (12%) over the nature tower but value per damage wise it is still less than the nature tower:
Tower Damage/cost=value per hit
Fire 824,520 / 2298= 367
Water 2,200,500/3360=655
Nature 1,935,000/2375=815
The flower tower on the other hand;
Tower rangexdamage=total damage value
Flowerx1 150x9090 = 1,363,500 682
Flowerx2 100x15050= 1,505,000 215
Flowerx3 90x909090= 81,818,100 3409
So the original flower is actually a lot weaker than a full nature tower and is also less value for money too, The second level flower is even worse, only adding an extra 150,000 damage (due to its decreased range) for a value of 215 per damage, seriously poor value for money (almost 4 times less than the nature tower).
The third level flower speaks for itself, hugely cost effective and extraordinary damage potential, the problem is getting there in the first place.
Anywho this post hasn't really added anything but maybe explained a few things hopefully.