End Game Classes

Since the best class is whatever style most fits the player, I think I will stick with Necromancer/Elementalist... Thanks guys

at the moment, most SCs and high end areas lean towards mesmer spikers, mainly for VoR and Keystone signet, although really, there is no best class since they are all used for different places, UW and FoW for example, wont use a single mesmer, but in DoA its unusual to find anything other than DWG ritualists or Keystone/VoR Mesmers.
The truth is, just find something you enjoy playing and stick to that, then try to find SCs that you can fit into. Anyway, since a-net will most likely nerf most of the prominent nuking skills-that could be seen as overpowered- in the near, or distant future (They have a habit of doing this), it wont really matter what you make, because they may well be useless in 3 months time. (not saying this will happen, just saying it might, so don't get to attached to a single style of play)

If I had to recommend a class, it would probably be Mesmer if you want a more agressive role, or a monk if you want a support role that will almost always be needed, I cant think of many high end areas that dont use a monk other than UW. Either way, as I said before, just find something you like, and do that.

There's not a single best class for general endgame, but there are some places that favor certain classes over others. And it has before and will again change, as the metas shift due to skill updates.

DoA currently heavily relies on ritualists and paragons. In HM the team very strictly is 4 ritualists, 1 mesmer, 1 paragon, and 2 monks. In NM any class except a warrior will do fine, however the monks and paragon are still required and the team usually runs ritualist heavy.
Genrally, DoA groups are caster heavy. Melee groups are secondary to whatever the caster meta is, and usually you can not find a physical PuG. You will should never see Elementalists used as healers here, as they can not make it through Foundry.

Urgoz & the Deep can take literally any class in my experience. Warriors and Assassins are desired as tanks, Paragons and Dervishes are generally undesired but can still find a team, and every other class is perfectly welcome for damage. Monks are required for healing, and a support necromancer with BiP and 1 hp is required (for party energy regen). The allowance of 12 man teams really opens up what you can have in your party, and these places tend to have the least strict requirements on groups.
Urgoz & the Deep should always be run in HM, imo. The difficulty to complete the missions does not increase by hardly anything if you're already used to being in HM.

UW requires two Elementalists as healers in recent Metas, you are unlikely to find yourself healing on a monk. I enjoy and see physways the most for UW, which usually means ~4 assassins or warriors and requires one necromancer. There are caster groups too, sit in ToA or sift through pvx for those.

The current solo farming meta is a ritualist with SoS. Dungeons/group farms like to use assassins and monks (but I'm not sure what else, not a fan of dungeons personally).
So basically as reaper said, there's no single best class that will take you through everywhere. But if you know you want to spend a lot of time at one specific place you can pick a class that is more likely to be desired in that location.