Well, you did say "powerful".
I agree with other posters that key comparisons depend upon level.
I like wizards--that is, I have fun playing them regardless of perceived or actual superior power--because of the endless fantasy fiction I have read involving them. The game rules support such--including the iconic Vancian "Dying Earth" setting from low magic to Rhialto level with Ioun stones, the Excellent Prismatic Spray, bound extra-planar servants, time travel, and so on. One story even indicates how having an anti-magic rune could protect from the most potent spells, which is what can in some ways and situations help fighter types.
But there is an intention for wizards as an archetype within a rule-based, probabilistic framework. The same with druids, a specialized type of cleric (nature worshiper).
I play druids when I want that particular flavor of role-play and tactics. Wildshape. Animal companion. Summon creatures. Turn nature against enemies. (last weekend in the group I DM, a 10th level druid commanded a battle [that is, overwhelmingly defeated the opponents practically solo] on an open plain--exceeding all other players including a wizard--by calling 5d10 vertical lightning bolts from the stormy skies while drifting airborne as a vulture, and supporting his advanced animal companion bear juggernaut charging into close combat on the ground, grappling and shredding.) But the "power" is in leveraging what is allowed and possible for the druid class, just as the power of a rogue in combat differs in technique and application from that of a strength or even dex-based fighter.
If power is the ability to effect change on any level from micro to macro-cosmic, then the rules (especially available spells and their effects) favor wizards at the highest levels. I recall the 1st Ed where wizards required the most XP to advance--that was for a reason.
I consider a high-level, well-understood (and creatively run), configured wizard with contingent, permanent, triggered, etc. magic (and golems, extra-planar servants, etc.) to be the most powerful with regards to effecting change on both small and large scales within the D&D rules system, and surviving in multiple situations in order to do so.
But I play what I feel like for fun in a given campaign, and multiple players are intended to run complementary characters, hopefully running what each considers fun, but also sufficiently powerful/capable within a given role.