I found her to be the most likeable female character on the Normandy 1 AND 2. Ashley is a bit of a xenophobe - understandable, given her history, but I couldn't groove to that. Liara during ME1 was a bit too breathy and unassertive and her personality, I thought, could be summed up in one word: Doormat. So I didn't gel with that either.
On the Normandy SR2, there's Tali, Miranda, Kasumi, Samara, Jack and EDI. Miranda's a cheerleader for the Illusive Man, so I didn't like her too much. Her character develops later, but she's still far too "Pro-Cerberus". Kasumi is a DLC character and a pretty shallow one after her mission is finished. Samara is a respectable character, yet, for design reasons, is naturally cold and distant (being a religious warrior/monk, that's natural). Jack is, understandably, extremely angry, but her random talk of killing people who didn't deserve it turned her off from me. EDI's an AI.
So Tali is the nicest, interactive female character. Plus, she was a member of the original crew - it's one of the reasons Garrus is so popular. Her story missions are also AWESOME, much better than Miranda's or Samara's or Jacks, because Tali's missions have lots of ROBOTS in them, and everything is better with ROBOTS.
I'm not a huge Tali fan. But out of all the romance options available to my Shepard, she was the best one. She's not my favourite character, far from it - I like Mordin and Legion and Garrus more than Tali. But since she is, in many gamer's minds, the best female romance character (with Miranda being a brainwashed sycophant, Jack being crazy, Samara being a naturally distant warrior, Kasumi being a weakish DLC character and EDI being software), there will naturally be a lot of fans who like her.