In my experience, with piles of different iPhones, as well as the perfect location to test low signal...here’s my assessments on Verizon and T-Mobile.
iPad Pro 11” - Worst reception by far, with no 3G support. In my low-signal test area it averaged 1 bar of unstable LTE on Verizon, no service on T-Mobile.
iPhone XS Max - Worst reception of the phones, but tolerable. Fairly stable 1-2 bars LTE, mediocre/unstable 3G. No service on T-Mobile.
iPhone XS - Better than the XS Max in my usage, but still poor reception. Very similar to the Max, but a little more stable. No service on T-Mobile.
iPhone XR - Best reception of the X style phones by quite a bit, mostly stable, and also tolerable on T-Mobile.
iPhone X - A bit better than the XS models, but still subpar. Usable and mostly reliable, but subpar. No service on T-mobile.
iPhone 8 Plus - Very stable, reliable connection. Occasional hiccups, but solid reception. Tolerable reception on T-Mobile.
iPhone 8 - Same as 8 Plus on both carriers.
iPad 10.5” Pro - Less than stable connection. Frequent connection drops, but usable. No signal on T-Mobile
iPhone 7 Plus - Similar to 8 Plus/8, but not quite as strong signal - I’d assume due to the full aluminum construction and lack of glass back. Tolerable on T-Mobile.
iPhone 7 - Same as 7 Plus.
iPad Pro 12.9” (gen 1) - Solid reception, stable, and fast. Tolerable on T-Mobile.
iPad Pro 9.7” - Same as iPad Pro 10.5”.
iPhone SE - While not the strongest or fastest signal of the iPhones, the SE takes the cake for most stable connection by miles. Not a single dropped call or connection on either carrier in my usage. Solid on T-Mobile as well.
iPhone 6S Plus - similar to the 7 Plus, but a bit slower.
iPhone 6s - Same as 6S Plus.