I like Regice the best because of the look even though back then I hate Regice because it killed my Blaziken with thunder and I need one hit to get the Brave Symbol. >_<
Regigigas, as it is massive, and its ability really adds to the effect of him being a titan. Plus gastro acid/worry seed/skill swap unleashes his power.
The case with Regirock which catches my eye, is how the Regi is constructed out of it's primary resource. You've got this ensemble of colossus boulders forming together to create one behemoth tank of destruction. I like.