Hard to say. Terrans do have some really cool units and you can relate more. Zerg are just fantastic, i tend to slightly favor them. Protoss are the most satisfying ones from a gaming pow (harder to master...) and anyway the lore is amazing. I would say that Zerg win by a small margin, but i cannot deny that there were times when this preference shifted from one race to another. Such a good game...
I certainly agree about Zerg and Protoss. I've had the most experience playing Zerg at this point and I find it much easier to build a sizeable army of Hydralisks, Ultralisks and Mutalisks, max out on upgrades and storm an enemy settlement with them. Also, I'm a Tyranid player, of course I'm going to enjoy playing Zerg! Protoss are also fun and the campaign story is particularly interesting for them, but the cost of their units and upgrades makes resource gathering harder, and their inability to heal their health is a pain. However I'm probably the one person who doesn't enjoy playing Terrans much. I don't know why exactly, but probably a combination of me being a die-hard fan of alien races, and their jack-of-all-trades nature stopping them from being as elite as Protoss or cheap as Zerg.
Yep, terran is the most boring faction to play (imo), but things as siege tanks and cruiser tickle my inner self. Plus nuclear strikes, drops and science vessels are satisfying when you manage to exploit them.