Don't know if it's the best opening line ever, but I always loved: A Prayer For Owen Meany was my introduction to John Irving. I absolutely adore that book.I am doomed to remember a boy with a wrecked voice - not because of his voice, or because he was the smallest person I ever knew, or even because he was the instrument of my mother's death, but because he is the reason I believe in God; I am a Christian because of Owen Meany.
The only John Irving book I liked so much I read twice. There are other I want to read again (the one with the "Wilzpatzer(?) Pension" chapter, is it 'the world according to Garp'?) but I got them from a library and never bother buying them. The few last Irving books are lackluster.
An ex-girlfriend got me onto John Irving. When I said I'd never read anything he'd written, she took me to Powell's in Portland and wasted no time finding me a copy of A Prayer... and A Son of the Circus. I will always love her for that! The World According to Garp is absolutely the book you're thinking of. It's another favourite of mine. My parents keep telling me I have to watch the movie adaptation starring Robbin Williams; I haven't gotten around to it yet. But I agree with your last statement. I wasn't impressed all that much with In One Person and I haven't read the latest one.