I say Zelda II. Maybe it's just me, but I prefer Zelda II because of how much of a challenge it was. ALTTP was also a challenge, but not as much as Zelda II. I also found Zelda II's music more memorable for obvious reasons :P
Also, it was fun discovering pretty much anything in Zelda II. ALTTP had more items and such, but I'd rather waste time on Zelda II. :)
i say Link to the Past only because i haven't played Zelda II. Plus, Link to the Past is challenging and i'm not saying that Zelda II isn't, i just haven't played it so i don't know if it's more challenging or not.
I haven't played Zelda II, but my brother owned LTTP from the very beginning. I love it, and in my opinion it's the classic Zelda game that you have to play to understand how strange and difficult Zelda is.
If you think about it, the first 3 games are sort of "linked" together. (Sorry for the unintended pun!) In the first game, you're looking for, what, the Triforce of Wisdom? You know Ganon has the Triforce of Power. Then in the second game, you're looking for the Triforce of Courage. We don't know what happens to the Triforce after Link uses it to wish Zelda awake. What if Ganon somehow got hold of all 3 pieces and then employed Agahnim to do his dirty work?
This is all in my mind, but I always thought it would be so fun if that were actually true. My suspicions were confirmed when I went into Ganon's secret room and saw the Triforce spinning there. Either way, LTTP rules.
Link To The Past set the bar in my opinion on what all Zelda Games should be like, as far as action and puzzles go on Portable Console Zelda games, as all Zelda games since then have been like Link To The Past, with the overhead view. Ocarina of Time seemed to set the bar for the rest of the consoles. So in my opinion, Link To The Past was much better. And in my opinion, is my 2nd favorite Zelda, next to Majora's Mask. And Zelda II can rot in flames for all I care.