Well you didn't add any caveats to the transformation and you said it's at will so... If mass is not conserved then I'd just turn into something heavy yet compact like a rhino and harvest infinite energy from the toggling. If I get support from a State I might even choose a whale so we can extract the most energy per toggle. You can basically build an engine from it, and since whales are like 200 tons and I'm 70kg, the conversion ratio would be pretty damn good. Though it would require a dedicated facility, which is why I'd rather go with a rhino or something, which is however about 100 times less efficient so it would require 100 times as much toggling. You get the picture.

Lobster.

I'd continue to live my normal life during the day, but transform every night to sleep, and don't age during that time. Hell, any time I'm not doing something that requires a human body, just push pause on the aging process for a few hours. Careful application could get me an extra ~40-50% lifespan for free.

The lobster's biggest problem is always having to grow new shells. If every transformation puts me back exactly where I left off, that could be an issue, but with a part-time human to stock my own tank/pool, getting enough to eat as a lobster to get the energy to do that shouldn't be an insurmountable hurdle.

Raccoon. Still got haaaaaands biiiitches

Hey OP, have you read the Animorphs book series? They're a YA series, but it actually gets into some heavier shit surprisingly (child soldiers, PTSD, etc) and are not half bad. I bring this up because the premise is some teens are given powers to morph into any animal they "acquire" DNA from by touching. Really gets into the mindset of each animal, what it feels like, how awesome (or terrifying) it is being each animal, etc. Give them a read if you haven't already.

Nobody said cats? I'm very disappointed. I'm going to comb my hair, look at you reproachfully, then go to sleep.

I am already an animal, but it might be fun to be another type of animal.

Look, in fucking July, my species was renamed the fucking paradoxical sea Muppet (Australyichthys paradoxus), and I’m kind of fucking sore about it.

Fucking Muppet! I’m a fucking seahorse. Fucking look at me you fucking imbeciles. Human fucks! Always fucking with you, fucking shit up, fucking up naming shit, fucking up the sea, ffs they’re the fucking muppets

So, “seahorse” is my answer, thx

90s Animorphs kid here. I've been considering this for 30 years.

And I'm not telling

A Rottnest Island Quokka. Highly promiscuous, well fed, no predators and beloved and protected by humans.

First thought is a very small insect, especially a small flying bug, so I could effectively vanish instantly and escape from most any situation.

Of course this comes with some assumptions, such as magically keeping my intelligence. And not aging at hyper speed while transformed, etc.

A raven doesn't sound too bad. Can fly, smart, doesn't really have a predator so I can explore the world at ease

A worm

I need to check if he's telling the truth

Bird. Daily commute will be fun.

Tapeworm. Just hanging out in a warm cosy intestine, protected from the outside world and food dropping on to me so I don't even have to move.

Do I get to turn back or is it a one and done thing?

If the latter, then probably a blue whale. I don't really want to live as a whale but I do want to know in my dying whale breaths that they'll never get the stench of my bloated 30 ton corpse out of the Australian parliament building.

When OP opened talking about hitting the hay, I figured his animal would be a horse.

A small and cute goat in a petting zoo. You get fed and you can headbutt people and kids to oblivion while everyone thinks it’s cute.

3 toed sloth is about my speed

Bear

Beaver. Any day. Favorite animal period. No doubts or worries at all.

I guess if you want a plant that would be fair game.

In that case, I would choose a quaking aspen based on the specimen named Pando. I would have a vast array of sizes at my disposal, from as small as a single seed to as large as the 106 acres that Pando covers.

I want to escape from somewhere? Turn into a seed. I want to destroy an environment? Turn into a giant network of trees.

Nothing deep sea, I don't want to deal with other creatures in an environment I can't transform back.

Nothing flying, heights would scare me.

I am between an elephant and a racoon. Probably a racoon.

See username. I could probably name a better bird for what I want, but hawk is literally right there. Not actually what it means in my username, but hey, if the shoe fits...

...as to why, it's basically just flight.

has existed

fuck yeah i'ma turn into a thylacine

i'd get so many scritches

Ancient Psychic Tandem War Elephant

Then I would go around abolishing the state.

I think a cheetah would be my go. Mostly chill, very cute, excellent at sunbathing, could still play with my dog, and very fucking fast sometimes.

yeah something flying would be nice

what's the apex bird?

Whale Shark.

Yikes, imagine having to learn to fly. Sounds rough. I'd be a different human that I would call Rex so I could go to the store without worrying about anyone recognizing me. I'd just rather not have to worry about seeing anyone I know when I'm not in the mood.

A tiny bird. Maybe a sparrow. Or an elephant, so that Iive in a forest and have a thick skin.

I'm thinking a goose, or a swan. Something where I get to fly but can also hang out on the water and just float.

Let's not forget Zeus turned into a Swan to have sex with the Queen of the Spartans. So that'd also be on the table.

a brain eating ameba if I can choose a host.

otherwise, a Giganotosaurus. go big or go home, right?

This dove:

I wanna fly away.

Wolf/husky.

Black bear. I could explore the woods around my house with my new bad ass senses. I'd be the toughest dude in the hood, no other animals could fuck with me. It's illegal for humans to shoot me and we all love our bears anyway. I could finally figure out where the deer sleep and track down the coyotes I've never got close too. Plenty of fish in the creek and the swamp it comes from and plenty of trash cans in the hood. I'd chill out on people's car hoods and porches just to freak 'em out. What a lovely life.

BONUS: If I see the redneck hunter out there that hates me, I'd follow him at a distance every time he sets foot out there. 😈🖤

Dhole (cuon alpinus). Wild canid in south/east Asia. Pack hunters that communicate with whistles and chirps. Barcode scanner dogs.

I always only have 2 answers for this question.

1. House cat. You’ll be pampered and have a comfy life. And unlike a dog, you don’t have to show affection to your owner. You can just not give a shit and they’ll find it adorable.

2. The only other option is some bird that can travel long distances and isn’t easy prey. That way you get to travel.

Golden Eagle. I could carry groceries.

Orca. Sure it wouldn't be useful on about 25% of The Earth's surface, but at least nothing in the ocean is gonna successfully fuck with me.

Cat.