Yup, I have narrowed it down to a few candidates already.
I'm playing Grunfeld against 1.d4 and 1.e4 e5, both of which severely limit my options.
1.Nf3 d5 2.d4 and it's a Queen's gambit
1.Nf3 c5 2.e4 and Sicilian
1.Nf3 g6 2.e4 and Modern/Pirc/Philidor/Dragon
1.Nf3 e6 is no use either, neither e4 or d4 lead to what I want
I have played 1.Nf3 f5 quite a lot in the past, I think it's ok but I want something a bit more robust now.
So I'm more or less forced to reply 1...Nf6! Now 2.c4 is far and away the main line and most annoying for Grunfeld players, because after 2...g6 3.Nc3:
We are committed to going 3...d5, (otherwise 4.e4 will lead to a King's Indian) which is not to everyone's taste.
So the line I am pretty sure I will go for is 1.Nf3 Nf6 2.c4 c5 3.Nc3 e6 4.g3 b6 5.Bg2 Bb7 6.0-0 Be7
I quite like the idea of these Hedgehog type positions, seems like lots of play for both sides while having quite a sound position, which is basically my objective when choosing an opening.
There are a couple of alternatives on move 3 (Nc6 and d5) which are worth considering, but unless anyone flags up any major issue I will go ahead with this line!