Which of these professions is the most helpful?

So I have a choice between: warrior, guardian and engineer. I can only pick 2 of these. My guild mainly does wvw but I like to do pvp and a bit of pve (living story, dailies, farms, etc) so anything that can do pvp well would be great. Which do you think is most useful in these areas?

Quite literally all of them, under the assumption you got HoT and/or PoF. In terms of WvW, spellbreaker is the bubble frontliner, required part of any zerg now. Firebrand continues the guardian main frontline tradition since day 1 and the scrapper is actually an amazing zerg healer now.

What your guild will most likey cry the most about is more firebrands.

Warrior's gonna be your best bet for doing all content. In PvE, everyone wants a BS Warrior for everything, and in farms you get participation for boons on them, and then they're fucking MONSTERS in WvW (bubble SB) and PvP(duelist SB). On top of that, they're easy to play and comparatively cheap to gear.

As a long time Guard main, I can only recommend it for it being super fun, but I will warn you that getting a proper setup for all game modes is gonna run you a higher money and/or more skill requirement than warrior.

Engi pros- Thematically is awesome, pretty cheap to gear, and arguably stronger than the other two for all of your content. Engi cons -not as sought after as a BS War in PvE or a front line support FB Guard in zerg WvW, for instance. Is gonna be the highest skill requirement out of the three proposed.

I think all of those are good, but they offer help in different ways and that should drive your choice. In otherwords, I wouldn't let the 'best helper' drive your choice for the class you want.

Any of them can do PvE.For large scale WvW Guard and War are better, for small scale all work, but in roaming War and Engie have better mobility.In PvP all work.In raids Engie and War are better, but War is more support and Engi more dps oriented.

Depends on what you have an what you are doing. If you have HoT and PoF then the clear best is difficult. I'd say Guardian is all round going to be the best bed due to firebrand and Dragonhunter but warrior is always useful with banners and can be run as its core spec and still be good. Engineer is an odd choice, but its healing build is excellent in WvW and fairly solid in pve. Scrapper does well in PvP. Overall Engineer is going to give you the least options though when compared to the other two. Guardian and Warrior both cover what you will need in PvE and WvW as well as being very good in PvP.

But ultimately that's up to you. Engineer is one of my favorite classes without a doubt.