That is a bold thing to say. Gladiator has a breath taking score. Especially those parts with Lisa Gerard's vocals are truly heart braking (i.e. the scene where Maximus comes home and finds his dead family).
being a soundtrack nut (scores really, I don't give a flying crap for soundtracks that have the latest bands on them doing their shit music for a movie, unless its music like Blade or such) and just having finished the score, I think it ranks up there. Nothing can really top The Battle and Barbarian Horde, but a lot of the elements in this score just keep going strong.
Yes, I like most of Harry Gregson-Williams' scores and the score in KOH was really good, but I think Zimmer puts a little more soul in his music. I have only seen KOH once and Gladiator maybe 20 times so I can't really compare their scores yet.
BTW, you being a soundtrack nut (like me), have you listened to James Newton Howard's work in The Village (one of the most underrated films ever, by the way). Especially the main theme "The Gravel Road". The violin in it never stops amazing me.