I tend to like ungulates, reptiles, and mythical creatures.
In fact, it was a dragon poster than got me into TF. It wasn’t even a transforming dragon, just a dragon breathing fire down onto a bridge. My dad was even worried it would give me nightmares. Yet, I just remember thinking “man… I wish I could turn into a dragon”.
Dragons, gryphons, hippogriffs, wyverns, pegasi…
I also like invented species TFs. Like, a TF into a Xenomorph? Awesome. Same with Turians, Deathclaws… Okay, maybe if they have a reptile slant, then it’s golden for me, heh.
Oh, and, I also like TFs into the smaller creatures, like mice, rats, rabbits, squirrels, etc. The size disparity between human to those is huge, and that’s always interesting.