It's like NWN2 in that regard. Addiotionally, many folks like to take 26 (or more) levels in the cleric class, since that means, their buffs can't be dispelled by other casters. Though in HotU this is probably not that important.
I think you can't do much wrong with just taking cleric levels. It's certianly powerful enoguh on it's own, strong. Maybe throw in a fighter level fot the weapon proficiencies and bonus feat.
My personal favorite would be a (human) cleric with two levels of fighter and barbarian each. This gives you martial weapon profiency, bonus feats that you can spend on e.g. exotic weapons, the barbarians running speed (yes, works in heavy armor) as well as the ability to rage, which can be useful early in your career. Four levels in high BAB classes give you an additional atack, as GawainBS pointed out (though some cleric players prefer to rely on the awesome "divine power" spell instead to boost their attack. I prefer to use both - normal attacks for easy foes and divine power for tougher fights).
Domain choices are strength and knowledge. Strength gives you earlier access to Divine Power
; knowledge gives you Identify
(personal preference - useful for some quests as well as a money-saver) and knock
, which together with the cleric's find traps
means that you can replace a rogue quite well.
That's actually the point of this build; there are more powerful ones out there, but I really like it for the campaigns because if offers a lot
of versatility: You can use any weapon and armor, disarm traps, open locks, and cast offensive as well as defensive spells.
If you want to take a look at some optimized builds (power-wise), you should check out the builds at the old [url='http://home.comcast.net/~worldofgreyhawk/buildsearch/']Epic Character Builders Guild[/url]. Lots of good stuff there. And you can always "tweak" one of those builds to suit your own preferences better.
Some examples are "The Fighting Ariel of Ehlonna: Rogue 5 / Cleric 29 / Fighter 6"
for another versatile build or "[url='http://home.comcast.net/~worldofgreyhawk/builds/data/build255551.html']The War Machine of Gruumsh: Cleric 26 / Barbarian 4 / Champion of Torm 10[/url]" for a melee-heavy, Divine Power