The Tali romance is pretty intense, as soon as Chambers tells you she has the hots for you, helping her deal with that crap trial, her father dying, and speaking passionately on her behalf in front of all her people? The captain is a greatly respected figure in Quarian culture too, and she really likes that you're her captain.
That's some serious stuff.
Honestly though, every romance has moments like that though, even LI's from ME1, which I'm looking forward to in ME3.
Miranda - Her romance is the most satisfying to me, in terms of winning her over. It can be like that if you're tough on Ashley in ME1, but that's more of you being "hard to get" than her. Miranda point blanks tells you to stop talking and focus on the mission early on. As the game goes on she softens up, and the loyalty mission completely opens her up to her humanity, which was crazy awesome. Niket betraying her, telling her to go talk to her sister, seeing her break down (in a good way), her thanking you. On top of all of that, this is the woman who relentlessly spent two years bringing you back from the dead, she's a damn polygon supermodel, and in the end she dumps the Illusive Man and Cerberus for you. When she says to not die, that was big too.
Jack - Her romance is tough for me to do because she's so lost. The more you learn about Jack, you feel pity and shake your head, but when it all comes to a head, underneath all the all-powerful **** bravado, she's a woman. A lonely, tough, independent woman who needs someone who gives a damn about her. She feels like she can put off the act with Shepard over time and when that happens, it's magic folks. I play a Paragon Shepard though, and she's more of Renegade Shepard's future sidekick in crime and main squeeze, especially the Earthborn background type. They'd both be orphans and would understand what having learning from the school of hard knocks is like. I'm surprised that didn't make it's way into the romance for the newer ME2 characters, but there's still room for it later on. One of those things they bring up and ask you bedside or something.
Kaidan/Ashley - This was GENIUS on the part of Bioware to make it hard to be faithful to your squeeze from ME1, hearing them question who you have become, calling your loyalty out, etc. Not entirely convincing as they're worked up and emotional about you, but still a tough pill to swallow. These are going to be super-satisfying in the events of ME3, no question, because of the anticipation. Of course, if you jump into bed with another, it could be disastrous and heart-wrenching. You know they're going to join your squad and you know Miranda, the most capable ex-Cerberus agent, Tali, your go-to Tech chick, Jack, who has nowhere else to go, will be there. The cat will be out of the bag on the Normandy when they pick up on your dalliance. I think if you've been cheating, everyone will jump ship, shake their fist at you, and you'll be on the extranet looking up Emily Wong before the final battle LOL. Hmm new LI. Anyway...
Liara - This one is very, very interesting. She's the innocent digger girl who can't shut up in ME1, bonds with you, her first experience in that department, and now she's a ruthless huntress looking to bring down the Shadow Broker. Fist smashing? Death threats? Damn girl! Bioware did not screw this one up, which some people seem to think. Paragon Shepard will be disappointed at first, but if the rumors about DLC involving Liara and the Shadow Broker turn out to be true, you'll get to see it come to a head. Redemption appears to be all about her getting in too deep fighting for you and your body after you die, and her revenge has consumed her. When you help her get it or maybe stop her, if you're paragon shepard and have her turn the Shadow Broker in, she'll be a woman who's seen and been through a lot lately and needs a hug and a hard... nevermind

. Not the awkward Prothean data disc girl anymore, and will be alive for potentially 1000 years to tell the tales of Commander Shepard as Benezia's daughter, who helped stop the Reapers. That's a serious legacy right there, having potentially the most respected future Matriarch in Asari history as your bump buddy.
BIG question - Who the hell is Miranda's father? No one names him, he's apparently a super rich Earth-based tycoon. Shadow Broker maybe? Wouldn't that be some crazy stuff if you have to help Liara hunt down Miranda's father? It's too much of a mystery not to consider. That is an utterly ticklish possibility.