To me, this is definitely the masterpiece of the great Mitsuru Adachi. I've read it over countless times and as many have pointed out, there's always something new to discover.
To me, the debatable ending is perfect because Hiro sacrifices himself in order to open up the eyes of Hideo and Hikari through the last ball that he threw in that semi-final match. As he loves Hikari, he gave his best to bring about what was missing in both Hideo and Hikari, for Hikari's happiness. However, the honey was already beside the bitter cake because Haruka was already waiting all along for Hiro and when Hiro's heart lost a significant piece, Haruka was ready to heal it. And so, the perfect H2 relationship.
What happens in that last ball is something that can only be understood after reading it over and over again (at least for me). As Hideo's straightforward honesty was tested, the fact that he wavered and missed meant that deep within, aside from all his other characters and loyalty to Hiro and Hikari, he couldn't take losing Hikari; something that Hideo only realizes himself when he swings his bat and misses... Meanwhile, Hikari has never seen and believed she would not see Hideo losing; something which she has seen in Hiro. It was only when Hideo finally loses, that the final piece missing between her and Hideo was sown. Something that Hiro had and Hideo didn't, was given up in that last ball. When the final ball was thrown, two pairs of eyes were opened, and one was closed in tears.
Adachi sieves emotions slowly and deeply so much so that I felt Hiro's pain as if my own. The loss amidst the friendship with Hideo and more-than-friends Hikari is not something that anyone can just take in easily. And that's where Koga comes in. Her patience and willingness to care for Hiro even though she knows Hiro's ongoing relationship with Hikari is the pillar that Hiro can lean on as he gives up a big piece of his heart.
And so, H2: Hiro Hikari was replaced with Hideo Hikari and Hiro Haruka... The heart-piercing, yet perfect ending to the series.