I like Season One the most because it and season 2 are the only seasons that seem to be an exact continuation of Angel, Cordelia and Wesley after Buffy 3. Plus there's Doyle, the Faith episodes, Buffy actually crosses over in this season, as does Oz and Spike. "In the Dark" remains my favorite episode of the show.
Season 2 is next, because the story arc was fantastic (I get shivers every time I watch Drusilla re-sire Darla right after she resorted to her human death and Angel promised to stay with her, only for her to be turned evil against her will). 2 ranks second because of Pylea. I was never really that enthused about that part.
Season 5 is third because, let's face it, one of the main reasons why so many people like it is because of Spike. But I like it for the Wolfram and Hart manipulation of the group, the tragedy, and of course Spike (hehe). And the finale was EPIC.
Then comes a tie for fourth/fifth. Season 3 has a great story arc involving Connor, Wesley, and Holtz, but some parts of the seaosn make me not want to watch it (The Angel/Cordy romance that appeared out of nowhere and was totally out of character for Angel). Season 4 is exactly the same way: amazing story arc, but is worsened by bad parts (Connor, Evil Cordelia). The main reason though, why I don't like seasons 3 and 4 is because Angel is WAY out of character. He's way too happy and smiling too frequently. He actually reminds me of his out-of-character attitude in the first two Buffy episodes.
So, to summarize, I rank the seasons thusly: 1,2,5,4,3. All five seasons are great, don't get me wrong, but 3 and 4 have many issues.