I think any core spellcaster is more complex than the fighter. Assuming by complex you mean something like "the number of options available at any one time". That fighter may have a bunch of feats, but for the most part the feats either apply or they don't. The wizard can have dozens of spells to choose from at any one time. And a cleric preparing spells at the beginning of a day, not limited like the wizard in the number of spells available? That's complex.
Complex to build actually, not complex to play. It can be tricky trying to build an effective 3e fighter because you are essentially given the choice of many feats (which you have to select from a list of many hundreds), and little/no direction on how you ought to proceed. So the onus is on you to sift out the good feats from the crappy ones and find out how they synergize with one another.
In a way, it could be even more unforgiving to play than the wizard, because the wizard can still change his daily repertoire of spells known until he finds a combination that works, unlike the fighter, who choices are effectively locked in (for better or worse).
But once you have it all figured out, it should be relatively straightforward to play, since it is basically a 1-trick pony (albeit a very good one) in combat.
