Liara is my favorite character in both games. Her biotics plus, well, weapons from other members equals quick enemy death. As for her having a baby, all we know is that the Asari take the DNA from whoever the meld with. We don't know if they can control it, such as flipping it on and off, or if it's something they have no control over. Maybe they have built in birth control that they can switch on and off. Or maybe Liara just didn't get pregnant that one time. Or maybe they used birth control.
As for my second squad mate, I play pretty much everyone, but Tali and Wrex are my favorite. Each squad mate has their uses. Ashley can soak up damage like a sponge and dish it out just as well. Kaiden was a balance between biotic and soldier. Wrex was another tank, good for shock trooper stuff. Tali could shut down entire enemy units if you played her right. Garrus was good as back up with a sniper rifle. And I've already covered Liara.
Mass Effect 2...my favorite character was Kelly actually. Then, Liara came back and she was my favorite again. But, neither of them are real squad mates so I liked using Kasumi and Zaeed. They seemed to be the best well-rounded characters, able to deal with any situation. Looking forward to seeing if they both come back on board for the third game, or if they'll just have cameos.