I guess my favorite would be a paragon. Simply because I like the concept of the profession - midliner that shouts to the team - motivating or commanding it. I quite enjoy the profession as it is, but it gets dull really fast. Even if you try to change builds up, it doesn't feel like paragon has good synergies from secondary professions. I mean there probably are, but they are so minuscule that its not even worth searching for them. With that being said, I like paragon, but it has quite a few limitations.
Aand that leads me to Ranger, my most played profession throughout the whole guildwars 1/2. I find it really versatile and adaptable to nearly every situation.
If you want to play melee, pick up some daggers or scythe and go nuts.
Wanna go ranged, sure, bow is there for you. Want to have a pet, man, we got you covered, go tame one and conquer the world.
Want to go support, take Rt as secondary and pop offensive or defensive spirits of your choice.
Want to tank, take but-loads of pcons and start the party.
Ranger is by no means optimal profession; I have to think really hard where ranger would do something better that other classes - interrupts, dps, support.. But its there, its definitely possible to cover a wide spectrum of necessities with a ranger. Therefore, ranger is my favorite.
Also, assassin is pretty cool. But meh. I often enjoy playing with a sin, but the profession seems one dimensional and super 'meta' like.