ME1 - I liked all the romance options, but I liked Ashley the best, with Liara a close second.
ME2 - I liked most of the romance options. I think I did chose Miranda the most often, with Tali being a close second. After Lair of the Shadow Broker, Liara probably became third. I was indifferent towards Thane, but I can see why others like it. I hated Jacob. I would agree with those who were disappointed with the character arc chosen for Liara.
ME3 - Liara was by far the most common, mostly because she seemed like the official romance. Hers seemed be the most developed and have the most content. Then there were the secondary paramours, Kaidan, Ashley, Garrus and Tali, which didn't seem quite as well done or polished. Then there were the new romances, Traynor, Cortez and Allers, which seemed tacked on and rushed. Then there are the forgotten romances, Miranda, Chambers, Jack and Thane. Finally, there was the aborted romance, Jacob. The vast majority of my characters ended up with Liara, simply because it felt like what the game wanted, although Tali was a distant second.
In the first two games, the romances felt fairly equal. In the last game, it definitely felt like there was an official romance which received by far the most attention. Sadly, Ashley, who was my favorite romantic interest in the first game was pretty unlikable in the third.