RF: As far as T’shuvos go, it depends on what style you enjoy. The Tzitz Eliezer will take you on a Mehalach through a Sugya, but often Paskens based on a Haskama or assumption of Achronim 300 years ago that the later Poskim rejected.
Most of R’ Moshe’s T’shuvos are not in what’s called the “classical style” of written T’shuvos. But there is nothing more L’ma’aseh for Americans than the Igros.
If you can handle Rav Ovadia, the Yabiya Omer is the way to go. Then again, no one alive today can handle Rav Ovadia.
All in all, I think the Tzitz Eliezer and the Igros Moshe are the most interesting for most people.
And if you have the right edition with the footnotes (it’s a sky blue 5-volume set, I think published by Hamaor), R’ Akiva Eiger is the best way to go.