Wizard by a little bit. The druid's spell list is by far the worst of the full casters. That's not to say it's bad. The druid is a full prepared caster with everything that implies and so, like all the others, just its spells alone make it stand head and shoulders above every non-caster class. If the druid had absolutely nothing but spells. If it was just a d4 hit die and a spell list like the wizard but with the druid spell selection I'd probably say it was tier 2. It has the same raw power of other tier 1 classes but lacks the flexibility of true tier 1 classes who can solve every problem instantly with just a few spells which they often don't even need to prepare ahead of time (in that most of its magic relates to nature at least tangentially so there could be some situations that if you didn't do even a moderate amount of planning you might not have the answer directly at hand). The wizard is Batman. The spell only version of the druid is Nightwing.
The thing is though, the druid doesn't have JUST spells. He has not one, but two entire extra classes worth of powers stapled to him for free. The druid's companion is as good as tier 5 character class. It can do one thing well and that's it. Usually that thing is fighting and while again, it's not that greatest at that, it's debatably equal to a fighter of the same level and having a whole extra class, even a bad one, for free is pretty awesome. Second, the druid has wildshape, which if all the druid had was that one singular ability it would be a tier 3 class easy. Wildshape gives the druid an absurd level of versatility to always be kinda suited to whatever problem the party is facing. It gives creative options to escape, to fight, to explore, and even to interact with NPCs.
So, let's add it up. Does a tier 2 class plus a tier 3 class plus a tier 5 class make a tier 1 character? I and most people would say yeah, it reaches that level of absurd ability coverage and dominance in absolutely every situation that tier 1 classes have. If you had to rank the tier 1 classes though I'd put it at a close second behind the wizard simply because while a druid can handle any situation the DM throws at them and dominate with ease the wizard still has this problem with accidentally wrecking the campaign with his world-warping magical powers. The druid is the ultimate answer for every problem. The wizard is the ultimate troll.