It's a tough one b/w the BTTF1 and BTTF2. I love their take on the future in BTTF2, but then again, I also love everything about BTTF1. The jokes, the wit and clever dialogues and the twists. BTTF3 is my least favourite of all, but still, it's a lot of fun to watch.
I say BTTF1. As they always say, nothing can beat the originals. But that's just my opinion.