Lets start off by saying this; Any class can be tankish. That people think the elementalist is a glasscannon is simply because they've jumped over to the forums, looked up the most FotM build they could find and whopdidoo, they've got themselves a elementalist that is somewhat viable, yet hilariously easy to kill.
This is a build that works silly well for elementalist sPvP/tPvP.. 0 AoE viability pretty much and definitly not for PvE, but you can standa fair chance against most classes with this one:
http://gw2skills.net/editor/?fEQQJAo...koJbTumkNNqYSA
As for the other classes:
Guardians can do pretty good damage, however for sPvP you can expect to be either playing a walking bunker or greatsword.
Necromancer: Well, they've got 2 healthbars, whats abit of extra CC?
Ranger: As pets are right now, you wouldn't rely on them in PvP at all.. the pets simply dont get enough hits in if people move around to make them useful; but its a fun class to play.
Mesmer: Dont go there. Really. Dont. Theres so many minor things with the mesmer right now. For example, completing 100% of the maps in PvE grants you a nifty golden star next to your name.. But your clones dont get it so suddenly your clones lost the ability to distract the opponent. Ctrl+T (Call target) ignores most of the skills mesmers stealth with, so if the mesmer stealths and summons a clone.. the crosshairs will still be over the real mesmer. Currently the mesmer brings nothing except portals to WvW sieges.
As you're interested in both PvE and PvP, and you want to be abit tankish.. I'd suggest a earth elementalist(Condition damage) for PvP and fire/Water for PvE, in PvE you wouldn't be tankish but you wouldn't need it. Or as Zito suggested, Guardian.. They're surprisingly fun.
If you just want to win alot in PvP, Pistolwhip or heartseeker thief is your best bet.