Borg is definitely a fav. I like mature athletes who accomplish 'cool' without seeming to try very hard. They are naturally 'cool' like Alain Delon or James Bond.
Borg definitely exudes this.
Federer has some of that, although he has a certain goofiness to him mixed with a regal presence. It makes him very likeable, but in a different way. Still, he handles himself fantastically.
I also like players who are funny/charismatic. Baghdatis is a fine example - he doesn't have to say anything to be funny; he's just full of life. That's why we love Ivanisevic as well. In terms of a more intellectual charisma Safin is fantastic - this is less about carefree innocence (or ignorance) of the likes of Ivanisevic and more about a flawed essence; you like the guy because he's a tragic hero. Other tragic heroes are McEnroe and Nastase - far less likeable but strangely admirable because they fascinate. That being said, neither of them suggest an intellectual aspect.
Some guys are funny unintentionally - eg. Boris Becker with the way he speaks, which is amusing. Lendl was uniquely funny with a dry sense of humour.
Some guys I admire for just being geniune, like Pete Sampras - he doesn't show a lot of charisma, but he doesn't go out of his way to change himself for the public's interest. He is a serious, down-to-earth professional. Rod Laver was as well. He comes to play and generally keeps his mouth shut.
Guys I don't like are posers. These are guys who try too hard to fit in - in spite of their insecurities. They are not bad guys per se, but they reek of bad attitude regardless of how much they work to cover it up.
Agassi fits that mold - he is beloved now but for years he tarnished his reputation by being a petulant a-hole. And he still is, despite the artificial persona he and Steffi concocted for him. It's very effective but very, very fake. He is not entirely to blame for this as his alienation is the byproduct of his difficult childhood. Roddick is also within this mold - he is not nice and extremely insecure in public. It's hard to sympathize with, although one can feel sorry for him. The one positive about him is that his honesty saves face in situations where one expects him to do more harm. This is also why McEnroe saved face.
David Nalbandian fascinates me - I think he's the most misunderstood athlete in tennis. No one can peg him correctly.