Onion Rings that stand out as being memorable since I first "discovered" this tasty side back in the 1960's:
Huot's Seafood Restaurant, Saco, ME
The Villanova Inn, New Castle, PA
The Plainview Diner, Plainview, NY (L.I.)
Hard Times Cafe, Alexandria, VA
(the original) Sonny Bryan's, Dallas, TX
Sonny P's Burgers, McKinney, TX
If I had to pick the all-time best it would be the stacked beer-battered rings at Huot's. Of course, everything tastes better with lobstah!
(I used to love Tony Roma's onion loaf but my last two meals at their Scottsdale, AZ and Georgetown, Grand Cayman locations were so dreadful I stopped patronizing that chain. Too bad; I enjoyed many good meals there in Orlando and San Juan, PR).