Well for me, the biggest stereotype has always been with Baptists. My mom spent the first 15 years of her life in a Baptist church and has always resented the denomination ever since. She recounts the straw that broke the camels back being when the cross-town Methodist church invited her church to a friendly softball game since both churches were holding their picnics on the same day at the same place.
The Baptist church actually took a whole church service to deliberate whether they should associate with "those Methodists" and ultimately decided "no". That, coupled with stereotypical attitudes of judgment, hypocrisy, and hate lead to my mom leaving and going to the C&MA church that my sister and I would eventually be raised in.
So that stereotype had been unintentionally engrained in me from a young age. My mom never went out of her way to criticize or disparage them, but if I asked her about it she would tell me the truth.
Other stereotypes I heard from my C&MA church about the church I now go to have been things like "they're the frozen chosen", or your garden-variety uptight WASP. There are remnants of that culture at my church, but it's so big that they're in the minority - and actually most of those "WASPs" are quite lovely, warm and friendly people, so... go figure.
Then on my dads side I grew up with some negative stereotypes about Pentecostals, since my grandma forced my dad and his sisters to go with her to an AoG church until he was 12-15. This was an old school Pentecostal church where they declared you weren't saved unless you spoke in tongues. It really wrecked my dad... he went atheist for about 20 years or so. There's a lot of stereotypes about that denomination that I get, both from him, and from my own experience with certain elements from that group: that they're often uneducated folk, no regard for anything rational or logical within the faith, are "holy rollers", and prefer something experiential to anything thoughtful, tangible and lasting.
So I may or may not have opened a can of worms up here by biting, but you asked and I answered.

I will say that I am calling these stereotypes, which aren't necessarily the norm or the majority. In some cases I've yet to encounter anyone that's convinced me the stereotypes aren't true, but in other cases I have.