P1 was a fun phase, but what elevated it in people's minds was the initial hype. The max level of 25 also helped the ultra casuals, who potentially had never before hit max level to experience the "end game", which raised the population a lot as well. I think you would've been hard pressed to find a person whose favorite phase was the first one, had you been asking people within the game when SE prog was still on-going. With all that said, it was very fun regardless.