Buffy and Angel may have been a true love :: inlove situation, but... there's always a but, and yes I have but face (like I am going to say but)...
Angel fell in love with her on sight, when Whistler showed him her... as a reason to leave his self pity. He knew nothing of her (other than her slayerness) before deciding he loved her. Many knock the Wesley/Fred ship as immature, but there is little in this world less mature than basing a decision like that on glimpses.
Faith and Angel had a true friend relationship going on. He was one of the few individuals who could understand the place she had gone in season 3 of BtVS. That's why he was the only one she felt safe enough to broke down to (a while after she awoke from the coma, she showed signs that she was done with the life she was living). They have much in common, but not real romantic feelings. Those she displayed were a more of a need to take something special from Buffy (who she viewed as having everything and appreciating nothing).
Now, we have Angel and Cordelia. They were friends though much good and even more bad (though they were never good friends until LA). She had seen the best and the worst of Angel (season 2 anyone... he put them through the ringer). He followed her to a demon dimension to keep her safe... and that was above and beyond procuring his conection to the PTB. They had great comradery and a wonderful friendship. As to whether she was the best for him, we will never know. What I do know is that they had the healthiest relationship (and that means alot more than many like to believe). True love is incredible and powerful and moving and intense... and it is not enough to secure a forever kind of love situation.
As Buffy said to Spike's comment about love being passionate and consuming... it will eventually consume all that are around it and in it (the love I mean). Plus, Angel was Buffy's first love (unless you are counting Pike, but not everyone's seen the movie), not true love. There is a huge difference that everyone here constantly ignores. First love is usually the strongest, but rarely is it the final (hence the name first love).
Buffy and Angel may be everyone's favorite, and I wouldn't ever dream of belittling anyone's choice, but it was also the most doomed of any other choice. Sad, depressing, sucky... but true. Spend a few years in the real life of adulthood and you will learn this lesson. Go into to it blindly and you will smack your head.
I am not saying that Faith and Angel or Buffy and Angel shouldn't be together. Personally, He and Cordelia had the most interesting (or realistic and best chance of lasting) relationship. But anyhow, next person.