I just thought it would be fun to see which forms most people liked, and perhaps in what order. My form ranking would look something like this:
Beast - It has great travel abilities what with the moving fast, the biting through enemies you don't want to deal with, and the ability to swim upstream, this form is the what's what of movement. My #1.
Energy - With it's sheer power and ability to target enemies in your vicinity (even offscreen!) this form has no equal in combat. Plus you can power it up with food to make it even more deadly! #2 for me.
Normal - This is the form that had me fall in love with Naija, plus it has all sorts of ADORABLE costumes! Plus you can sing which has a number of uses. Did I mention ADORABLE?

This gets #3.
Dual - This form's power exceeds even that of the energy form! You get it really late so there's not much chance to use it but it takes out any kind of enemy with above-average health in a heartbeat. And that scream is an amazing room-cleaner. #4!
Fish - Fish is more like my "travel fast when I don't feel like playing the game" form for just avoiding enemies and getting from Point A to Point B. It's a great form for what its uses are...but it's kind of boring ya know? #5 I say.
Spirit - It stops time, gives you a breather during tough situations, and it absorbs energy shots for health! That's just awesome! And it gave me a way around those nagging blue crystals that I had been wondering about the whole game. My only complaint is that I wish there were MORE blue crystals so I could feel like I needed this form a little more. It just didn't have enough uses...so it comes in 3rd to last at #6.
Sun - I have no strong feelings one way or another about Sun form. It has zero use in combat, but it lets you explore new areas. It's about as much of a "blue key" as Spirit form is (though slightly less blue). The reason I rank it lower than Spirit is the fact that it doesn't even have the shot destroyer built in like Spirit does. Perhaps if it had some sort of damaging ability I would rank it higher.
Nature - I love the look of this form but man was I so dissapointed with its powers...Does anybody else think this form is totally useless? I like to spend most of my time fighting in open waters (not the area, the type of waters) swimming around, dodging, and attacking. But Nature Form is #1, hard to aim as its bolts have a curve to them and #2 limited, because it forces you to attack enemies close to the walls. If it had an attack that could damage enemies that don't necessarily stay near walls it would be alot better. I was just trying it out in the Kelp forest, and boy all the times I tried to use it it was more work positioning the thing to actually hit the swimming enemy than it would have taken to just shoot it with energy form. Especially the spiders and Dragon Seahorses. This form comes in dead last for me. Plus I'm good enough at platforming that I didn't even use its boosting ability. Total loser in my book. #8.
Those are my thoughts on the 8 forms in Aquaria! But I'm interested in what everybody else thinks. So come on forumgoers let's hear it!