Tuneonmusic's Easy Cover provides a "beautiful yet simplified" version with free downloadable MIDI and sheet music.
Anime Piano Tutorials' Beginner version focuses on playability without requiring deep music theory knowledge.
The Zankyou Sanka arrangement by Animenz is widely considered one of the best for its "Japanese-style" ornaments and faithful replication of the original song's percussion rhythms. You can also find a Synthesia version by AkiPiano that transcribes this specific performance.