The Cleric is the "strongest" character, but the Druid is very close, as are the Wizard and the Ranger.
Cleric shares the best non-Fighter-type BAB and Hit Dice progression with several other classes. Clerics also share the 2nd best Save progression (2 good and 1 bad) with 3 other classes. Only the Druid, Monk, and Ranger have as good or better combination of BAB, HD, and Saves.
Clerics have a bit of disadvantage with weaponry, although the preferred weapon can help overcome that a bit. Plus, the armor pop they get is even with (or better than) some of the fighter-types. At this point, NO OTHER CLASS can say they have a more favorable combination of the traits discussed so far: BAB, HD, Saves, weapons, and armor.
Clerics are strictly average with feats, but only the Fighter, Monk, Ranger and Wizard can claim more (either actual feats or virtual feats). Clerics are also among the dregs of skill points. Here, the Monk and the Ranger begin to gain back some of the ground they lost.
Clerics do not fare so well with special abilities (EXCLUDING spells), as they only get the Turn / Rebuke undead ability. 6 of the base classes get a larger, more "powerful" ability set, including both the Monk and the Ranger. However, it is important to note at this point the limited environment of the Ranger. Many of his pops occur in specific environments and/or against particular opponents. Since those are campaign specific, the usefullness of these abilities may vary. To a lesser extent, the same is true of the Monk. Much of what the Monk does is close-combat related. Obviously, combat is likely to play a significant role in most campaigns, adventures involving few humanoid opponents will lessen the effectiveness of the Monk's ability. Certainly, the Cleric's undead power is limited in scope, but this just serves to prove that Special Abilities may not always be a sure sign of "power" in all instances.
Lastly, we turn to spells. Wizards have a larger selection of spells, but a Cleric will have access to more spells at a given instance because they have few limitations on the spells available to be memorized. The spellbook tips the balance of spells favorable for the Cleric. Further, Divine spells are not effected by armor as Arcane spells are. Frankly, spells cast behind an AC of 16 or 20 are better than spells cast behind an AC of 13.
Druids and Rangers can be more effective in a limited environment, and in some campaigns they would be the most powerfull classes. But, if we strip away magic items, campaing considerations, outrageous ability scores, racial bumps, etc. and just look at the core classes, the Cleric wins due to the combination of shear bump and versatility.