In Jain tradition, there is no single universally fixed Tirthankara who is officially called “the most generous” in all their previous lives.

Many Tirthankars performed great dāna (charity) and tyāg (renunciation) in earlier births, and different texts praise different lives for different virtues. So it is better to say:

• Several Tirthankars were अत्यंत दानी and पुण्यशाली in past lives
• But Jain scriptures do not name one clear “most generous” Tirthankara for everyone

If you want, I can tell you which Tirthankars are especially प्रसिद्ध for dāna in their previous lives according to Jain stories.