When Tai is introducing the characters he introduced Sora as "pretty cool, for a girl" implying a crush. (Dub only)
Sora is very glad to see Taichi returning when he returned during the fight with Tyranomon in Adventure episode 18.
Taichi breaks down after being unable to rescure Sora in Adventure episode 19.
The largest evidence comes from the movie "Digimon Adventure: Our War Game!" (made into the second part of "Digimon The Movie 2000" for the dub). Taichi is writing an apology letter to Sora. Hikari playfully adds a heart to the end of it and sends it. When Sora receives it she is happy to see the heart.
In episode A very Digi-Christmas, in Digimon Adventure 02, Taichi was about to confess his feelings for Sora and ask her out after the concert, without know that she baked some biscuits for Yamato (English dub only).
In the movie Diaboromon Strikes back, a wind gust blowed in Taichi, Sora and Yamato's direction. Sora protected herself behind Taichi, even if Yamato, supposed to be her boyfriend, was right next to her.