HAHA..This question is really hard to answer because I think any guy is suitable for SH. Why can we ONLY pick 2? That's so hard...but here's my pick:
YES YES I LOVE LOVE LOVE GEUNSHIN so they will always be my favorite but I would love for her to pair with:
1. LEE WAN - I know he is in the army now and he won't be out until 2011, but I was so mad that they didn't share any kiss scenes in Tree of Heaven because SH was too young. Tree of Heaven had so much potential for steamy kiss scenes between SH and Lee Wan. Now that SH is mature now, I would love for them to repair and "complete" their kiss scenes. I know the chances of this happening may be slim since this would be their 3rd time repairing. Also, I would love to see them in a lighter drama rather than in melodrama like in Tree of Heaven and Stairway to Heaven. Before GeunShin, I was very much into the Lee Wan-Shin Hye coupling. Back then, it seemed like their age difference was huge since she was 13 and he was 19 but now, its not too bad since she is 20 (almost 21) and he is 26.



2. BAEK SUNG HYUN - Yes, her other co-star in Stairway to Heaven (the young Kwon Sang Woo). This guy has grown up to be soo hot now and he is pretty good actor too. I would love to see them in a drama now that they are mature now.
On a side note, for those keen Geun-Shin analyzers, I am confident you could write a essay on them. the thesis being "Geun-Shin as a couple."