Necro or Guardian
Both are fun, guardian is more appreciated in group. Necro is a selfish damage dealer class, whereas guard is a support-ish respectable damage class. Most people will advice guardian, because necros are lacking boon share, invulnerabilities, and vigor access. For spvp necros aren’t in a bad place in group, but they suffer from being focused.
edit: typo
Well guardian is by far the more welcomed in groups, including dungeons. In the role of supporting a group, they’re much better than necro.
I have one at 55 and have run a necro to lvl 30 or around that level before stopping it. I personally found guardian more fun for a number of reasons:
1. Necro weapon skills are pretty boring and aren’t that great.
2. The design of the necro class forces one to commit their build and utility slots to either power and damage with wells, or to condition damage and spreading them, or to minions. There’s too much separation among these three necro styles and the skills and traits don’t really synergize well with one another. Whereas in a guardian almost all skills and traits can contribute in some form to whatever playstyle you pick, and the weapon skills are fun to use and are quite useful.
In pvp I hear both classes are great. I don’t play much pvp so can’t comment more than that.
EDIT: You really should try each class and get a feel for them yourself. That’s the only way to be truly sure.
If you have a dedicated group or guild you know you will run with, then Guardian is a good choice. The Guardian is designed as a support class and works like the Paragon did from the original Guild Wars. I.E. good in groups, but not so great solo. They also suffer quite a bit against ranged combatants. The only thing easier to kite than a Guardian would be an actual kite. 
If you are going to be doing a lot of solo play or solo queues in sPvP, then Necromancer is definitely a better choice as they are far more self contained as a profession than the Guardian is. They can do well at range or close up and have far more health than a Guardian has. They also have more choices when it comes to traiting their profession.
2. The design of the necro class forces one to commit their build and utility slots to either power and damage with wells, or to condition damage and spreading them, or to minions. There’s too much separation among these three necro styles and the skills and traits don’t really synergize well with one another. Whereas in a guardian almost all skills and traits can contribute in some form to whatever playstyle you pick, and the weapon skills are fun to use and are quite useful.
The Guardian really only has two specs to choose from. You either go Meditations to be a glass-cannon DPS, or go with Shouts to be a tankier support class. As the Guardian is designed as a support class, the Meditations build is usually only used in PvP.
As a side note, you can create both professions and once you are done with the tutorial chapter, you can take them to the Heart of the Mists where they will be level 80 with all their skills and most of their traits unlocked. You can experiment with both of the professions there and see what each can do. There are some “spirit” versions of each profession to fight and some of the harder boss-type MoBs there that you can have a look-see with each profession.
Thanks.
“You can get more results with a kind word and a big stick,
than you can with merely a kind word.”
Amongh my characters, I’ve got a guardian and a necro that I leveled to 80. And between the two, I far prefer the necro over the guardian. I tend to play solo a lot so the necro’s more selfish characteristics suits me. I say the same thing about my ranger too. Both are very self sufficient classes that tend to handle most things quite well alone. If I go for parties in fractals or dungeons, I use my warrior and depending on the group, I might make him very selfish as well.
And before someone asks, all of my characters are kitted in full exotic or a mix of exotic and ascended gear.
fractal + dungeon – guard
wvw + pvp – necro
I have both and I think Guardian for PVE and WVW.
PVE – boons, wall of reflection, healing tome (elite skill)
WvW – Staff skill 3 and Retreat for speed buffs. Staff skill 1 to tag all the mobs to get more WvW experience.
Guardian for PvE and WvW. Necro is very situational in both and a bit more suited to sPvP.