It depends on what you consider magnetic...
For charisma and sex appeal, I think no aspect can top Venus-Mars conjunctions, especially in Fire signs. They simply have an aura of power and animal sexuality. If they have a target, they go and get it. Other women and men are crazy about them, and the Mars-Venus person tend to "care a s**t" about other people's opinions which makes them have this powerful energy. They exude self-confidence through they mannerisms and these men are very macho-like. Women tend to have a masculine demeanor but the kind that doesn't turn off others.
The square is similar, but their demeanor is much "weirder" than the conjunction. Seems like they aren't comfortable with this strong energy. The conjunction knows EXACTLY what it wants, the square is torn. They have magnetism but it's not so much "in your face" as the conjunction.
For charm, definitely Venus-Jupiter aspects, with the conjunction being the strongest. These people are jovial and easy-going, and generally people enjoy their company a lot since they make you feel important and valued. They are generous to a fault and always lean a helping hand, again, especially when the conjunction is in fire signs.
Sun or Moon trine Ascendant are other charming aspects. With the Sun, you appear very sincere, good natured, full of good intentions and self-confident if Saturn isn't in the mix (square/opposition/conjunct sun or moon). With the Moon, you appear nurturing and affectionate, but maybe a little moody.
Neptune and Uranus contacts with ASC, DSC, Venus or Mars make the person independent, original and eccentric with Uranus and difficult to read, almost ethereal with Neptune. It's very intriguing. Neptune gives the appearance of an innocent beauty which is very appealing.
For dark magnetism, Pluto is number one in any aspect. Sun-Pluto, ASC-Pluto, Moon-Pluto, Mars-Pluto, Mars in Scorpio, Moon in Scorpio or heavy Scorpio in general. They exude self control and confidence. They have this "dangerous" aura which incourages the other to surrender to the person, a la "take me and do what you want with me". It's much less apparent than Venus-Mars but people perceive it. They are mysterious and seem trustworthy in some way and people tend to trust them and tell their best kept secrets spontaneously. People don't really understand why, but they are attracted. The scorpionic/plutonian piercing gaze is a big factor in the attraction.
Mercury-Pluto can seduce with their voice, which almost always is deep and/or powerful even if they whisper. Its tone resonates across the room and when the Mercury-Pluto person speaks, everyone shuts up and listens, also because this person doesn't tend to speak much unless he/she has something meaningful to say. Their words are carefully paced, controlled and powerful and the subject is not afraid to say what he thinks even when it's uncomfortable or brutal.
Calmness and pragmatism is also magnetic for some people. Here is when Saturn comes in. The Saturn person is calm and collected, a problem-solver and only a few things shake them. They tend to be sarcastic and have an acid humor. Some are intrigued, some are just annoyed because they tend to be pessimists and to complain if Saturn is not well integrated.
Remember that other aspects to these planets/angles modify the energy.