It would depend entirely upon the specifics of "had to;" are atheistic churches allowed? Would I be living under a theocracy, so I basically have to choose a specific religion or face persecution? Or is someone putting a gun to my head and giving me a choice of church or death? Because in that latter case I'd say pull the trigger.
Hypothetically speaking, I'd probably join a branch of Satanism like The Satanic Temple. They're wholly atheist but use Satan as a symbol for individualism and free thought. I'd rather fight against brainwashing and indoctrination than passively support it.
If I had to attend? You mean a gun to my head kind of "had to attend"?
It'd be the LDS Church hands down! Now that I see it through the lens of atheist eyes, the shit that Mormon's do, say, and believe is some pretty high comedy. It is without a doubt one of the world's dumber religions.
I joined a UU church because a) atheists are welcome, as is everyone else, b) there is NO dogma, nothing you have to say you believe, and c) primarily because of their RE program, where kids of all ages are taught to think for themselves. They are taught about the world's religions (and celebrate many various religious holidays--Christmas, Kwanzaa, Hanukkah, Dwali, etc.), taught what those religions believe, but never ever told "this is true or this is right." Except for the 7 UU principles, which boil down to: everyone is worthy of love and respect, be kind to animals, be kind to the earth, recycle, trust your conscience, stand up for others, etc. Oh, and by the way, UUs are VERY open to the LGBTQ community, which is important to my family, as two of my grandkids are trans(very recently out), and open only to their friends, and at our UU church. Not at their workplaces yet. That alone should tell you how much they trust the people there. We're also really big in social justice stuff.
The reason I went there to begin with was because my grandkids were living next door to an Evangelical Christian family, and the girl in that family was telling my granddaughter that she was going to hell for reading Harry Potter books. My granddaughter had not heard of this "Hell" before and was worried. I told my daughter we needed to get the kids some religious education so they wouldn't be susceptible to that kind of terror tactic, or to members of cults who come knocking on your door and promising you happiness if you only join them, and she said, "If you want to take them someplace like the UU church, go ahead--I will personally never set foot in a church again." So--I started driving 60 miles each way every Saturday to bring them to my house, keep them overnight, take them to the UU church in the morning, and then 60 miles back to their house. But it worked. And in the process, we all became UUs. About half our congregation is atheist--the others are a mixture of Buddhist, liberal Christian, Hindu, Wiccan, agnostic, humanist, etc. We like it. :)
Baptist. Just because I really love gospel choirs. And their meetings are so high energy. I wouldn't believe or agree with much of what they preach, but it's highly entertaining, and that's the only way I'd like church.
Unitarians!! Secular buddhism is great too. The salt lake unitarians are wonderful. Unfortunately I can't go as often as I'd like because I still schlep my reluctant self to no church to appease family.
Even if they can be flaky, I would join a pagan coven again, if I had no choice. They know how to have a good time, being sky-clad and all that. Or the Church of Satan, considering they're just a bunch of atheist agent provocateurs that enjoy shining a light on religious hypocrisy. That's always an entertaining way to spend an afternoon.
I considered the Jains for awhile because they are anti-killing (ahimsa.) Not that there are any Jains around where I live.
But they do go a little over the top with the broom to sweep the bugs away and masks over their faces. Still, I could deal.
However the kiss of death is the no attachment idea that is so over the top with not even being attached to your clothes or hair (have to pluck it all out when you become a nun or monk.)
And I realize all religion is so utterly stupid, I don't want to be a Jain.
Also don't want to be a UU because they are WAY too respectful of other religions which I consider totally stupid and delusional. And I think the UU are a little too much into the name dropping with their Henry-David-Thoreauing all over the place.