It's more the creative force in the band. In Iron Maiden, it's the bassist. In Dream Theater right now, It's the guitarist and to a lesser extent, the keyboardist. It isn't the instrument, it's the creator.
^Pretty much this.
But trying to put this in a context where we can actually debate/discuss it: If we are talking about rock/metal bands, and talking about
performance of the music itself (vs. the creative vision and
creation of the music), I would say it's a tough call between vocalist and guitarist. For the vast majority of songs, the guitar riffs, chord progressions, and solos, and the vocalist's singing are at the forefront. Those are what people pay the most attention to. Those are what people remember. Those are what people focus on primarily and remember the most. When you hear a song for the first time and are forming that initial impression about whether or not you like it, it is usually one or both of those. Unless there is a very specific and unique thing the bass or drums (or keyboards) are doing, that isn't what stands out. I
can think of obvious exceptions. For instance, as a young kid, when Another One Bites the Dust was all over the radio back in 1980 or whenever that was, it is obviously that very prominent bass line that grabs your attention and is the main driving force in that song. But, again, that is the exception rather than the rule. Even in a band like Dream Theater where you have virtuosos like John Myung on bass and Mike Portnoy or Mike Mangini on drums, on most songs, for most people, the
immediate hook will be the guitar or the singing. You usually notice the drums, bass, and keyboards later, or at least, to a lesser degree.
Having narrowed it down to guitar and vocals, I'm really hard pressed to pick one over the other. It really depends. But if you put a gun to my head and made me choose, I'm going to go with vocalist, for basically two reasons. First, you sing along to the vocals, which makes them stick in your head. Even if you don't know the words, how many of us have caught ourselves involuntarily humming along to a vocal line in a song? It is infectious and hard NOT to do when a song speaks to us on any level. I remember a quote from Ritchie Sambora a long time ago, where he was saying that the best, most effective guitar solos were the ones that weren't necessarily the ones that had mind-bending shredding going on. They are the ones that are melodic that make people involuntarily hum along. That quote about his philosophy as a guitar player has always resonated with me. And I think it really speaks to the quality of vocals in any given song. Second, more often than not, the singer is a HUGE part of the recognizable signature sound of any given band. Even people that have a terrible ear for music who could never recognize a guitar player's signature sound can usually recognize their favorite singer by voice, even if presented with a new song they haven't heard before. There is something unique about the voice that an instrument cannot fully capture.
So, yeah. For what it's worth, my vote goes to vocalist.