I really enjoy the
Cavalier from complete warrior though you won't be able to enter it until 8th level and it'll pretty much suck up all your feats to qualify (all mounted feats). Still a fav though as some very neat stuff comes from it: deadly charge/day, improved wpn focus (lance/sword), and the best of all are the levels of Cavalier contribute to a paladin's special mount progression. I'm a sucker for the mounted knight with shining lance battling the dragon sort.
Knight of the Chalice doesn't provide any synergies but is fun for a crusader against evil outsider paladin-typen and
Knight Protector is another fav of mine because it's so fitting to the knight in shining armor paladin-type, not to mention the special class ability supreme cleave at 3rd lvl.
As for Complete Divine:
Hospitaler is pretty good, you lose special mount progression and staggered spell progression in return for continued lay on hands, remove disease is still had by 3rd, and you get bonus feats from the PrC every 4 levels starting at first. Pious Templar gets it's own spell-casting, good Fort/Will saves, full BAB progression, DR/-, weapon specialization later on and some bonus feats. Mettle is had at first level and is the fighter-type's version of evasion: magical effects that have Fort/Will saves partial instead suffer no effects of these spells at all on a successful save.
I haven't really looked at Divine Crusader, though it looks like it offers little except some strange spell-casting in a single domail spell list, all the interesting stuff is had at the PrC's higher levels. Wouldn't recommend it at-a-glance.
All in all I'd probably mix and match believe it or not between Pious Templar and Cavalier for some interesting combinations, but that's just me. Have fun!