My first TF (and most frequent probably since) was to become a dog. Think I’d seen Time Bandits, and someone told me they wer going to turn me into a dog. I thanked them and felt myself slowly ducking down onto all fours as my arms became forelegs and my hands became paws.
Only lasted a few minutes after that on the first go, but boy did I enjoy that few minutes.
So I’d recommend dog, but many other animals are good, and any transformation gives you a whole new perspective, physically and mentally