The reason I want to do this is because my DM is being mean to a few of the party members for "how basic" their characters are, and he has also TPKd us several times. So using polymorph, I want to walk into the big bad evil guy’s castle, and kill 'em.
(My party is level 8)
The classical high end beast for polymorph is the Tyrannosaurus Rex, as it can freely move around on land, has a lot of hp, and a nasty bite. Conveniently, it also is CR8.
Alternatively, especially if you need something with more intelligence that can carry out a simple plan, you can use the CR7 Giant Ape.
However, don't expect that this will allow you to stomp over the villain easily. A party of 8th level should be stronger than a T-Rex, because a CR8 creature is just one of several level-adequate encounters they can expect to successfully defeat each day. So if the villain is a challenge for your party, they should be able to put down a T-Rex, especially in their castle with all their minions and supporting fortifications and siege weapons.
Even more fundamentally, this does not look like an issue you will solve in-game. Your DM has unlimited leverage to out-do whatever you come up with. To me it sounds that there are issues with how they run the game, killing the entire group several times over. Unless that is the game you signed up for — and it does not sound like it — you need to talk with them and make them aware that how they treat the players and run the game is not enjoyable to you. In the end, the DM does not have the ultimate power. The players can always opt out of the game and find another DM, if you cannot work out some solution that works for both sides. That solution can only be found by talking it over out of the game, not by polymorphing someone into a T-Rex or any other in-game means.
The primary limitation of polymorph is that you can only take beast shapes. With that said, in this range there are two shapes that have similar AC and hit point totals, with the variation being mostly tactical in nature- but that's not something you should overlook.
The T-Rex has 21 less HP than the giant ape, but moves 10 feet faster on the ground -> It has no climb speed, however, and it can't focus its multi-attack on a single target.
The giant ape has 1 lower AC (12), but it's also a biped and you could hypothetically look for a way to get larger armor to wear on the form, including paying for what would normally count as "barding." This means your giant ape with two swings of +9 can focus 40+ damage on a single target with 18 AC if you're willing to drop a chunk of gold or a valuable item for it- and it also has a +9 to athletics for actual grapple lockdowns (athletics vs. athletics/acrobatics as opposed to having to hit AC that might conceivably reach 25+) with a 10 foot reach - one that is as hard or harder to escape than the T-rex's 60% of the time. As the final cherry on top, the giant ape can actually competently grapple creatures above medium size, all the way up to gargantuan, so an enlarge spell on the BBEG won't nullify half of your main attack.
It's also got a climb speed and a ranged attack worth 30 average damage. In most situations where I'd consider a T-Rex, I'd consider the giant ape superior.
Edit: In fairness, you could pull the barding trick with the T-Rex, but the giant ape can also hold a shield...
But I do agree with Groody. The real problem here seems to be an atmosphere of condescension from DM to Party, and that sounds like dissonance between game expectations. A conversation might fix this, or your group might be better off looking for a different DM that accounts for the dynamic of different player skill level within their game rather than mocking them for a lack that can be learned.
For comparison:
Giant Ape: AC 12, hp 157, attack bonus +9, damage 44
T-Rex: AC 13, hp 136, attack bonus +10, damage 53, auto-grapple Medium with Restrained, must attack two targets
Huge Giant Crab: AC 15, hp 161, attack bonus +9, damage 54, auto-grapple anyone
I agree with the other answerers that this will not solve your problem with your DM. You need to make sure your DM knows that it's not fun for you when the battles are this difficult.
I also want to note that using polymorph on yourself is usually a bad plan, because you will lose concentration quickly when hit. You lose the benefit of your feats (such as War Caster) when polymorphed.
