Cool thread Boydie!
I guess as I prefer lyrics that lack pretentiousness I veer towards opening lines that can say something quite profound in a really simple and almost effortless way. I've spent a while mulling it over and I'm not sure I can think of a 'favourite' per se but there are some that I think pack quite a punch and set a certain tone without seeming to try too hard. A few examples:
Cat's in the cradle - 'A child arrived just the other day, came into the world in the usual way, but there were planes to catch and bills to pay, learned to walk when I was away'.
Your song - 'It's a little bit funny, this feeling inside, I'm not one of those who can easily hide'.
For me these say a lot without really saying very much.
A more modern example is from The 1975 Girls, though this one has tried a little harder I think - 'Bite your face to spite your nose, 17 and a half years old, worrying about my brother finding out, where's the fun in doing what you're told'.
I guess my 'favourite' is one that gets me every time I hear it and makes me think 'f!"k that's clever, wish I'd written it!'. Beautiful South Perfect 10 - 'She's a perfect 10, but she wears a 12, baby keep a little 2 for me'.
In terms of self written opening lines I dunno really. One that I've always liked for no reason other than it's a bit absurd and doesn't seem to go anywhere when divorced from the rest of the lyric is 'I been cursed with a silk tongue that could charm the pants of the virgin Mary, I smashed my face of every branch fallin' outta the good looking tree'. Actually came from the odd throwaway remark or two when having banter with mates... oddly enough the lyric has been rewritten entirely except for the first verse.