Tank
Dps
Healer
Doesn't matter
Healer or Ranged DPS, as it stands, probably a healer since they're likely to be communicating cooldowns anyway and are the role that should be watching players already.
Tanks can't see the whole fight with their back to a wall, forcing their camera into the boss - and Active Mitigation makes some of them require much more focus. Melee DPS are in a similar position, mostly because they actually have to move out of shit and stay on the target when a tank moves, whereas ranged have it laughable easy compared and have what feels like forever to react to shit coming towards them, and like the healer, can actually see the raid.
The arguments regarding "tanks steering raiding" aren't very useful for guilds progressing in a boss for more than 20 wipes. This is because after a point the steering is no longer required to be called and it is assumed by everyone, even the worst players around, and what mainly becomes important in leading is calling out for important changes that must be done and can not be preplanned. At the very most you also have someone calling out the next encounter ability so people are prepared but nothing that wasn't planned because that would mean something is wrong with your strategy if you need a tank to steer the boss differently every time.
For me, healer, maybe because i´m used to it, ranged DPS is almost the same to me as for raid leading, but melee or being a tank... a lot of stuff happens at melee, so i get easily distracted doing the mechanics or trying to maximize DPS.
I think that even in the case that some roles are better to have an overview of the encounter, it boils down at the player and how used is to his/her role and being RL.
Personally, I liked doing it as a healer.
But people who can raid lead and still do their dps quite well amaze me, cuz I couldn't do it worth shit.
I believe everyone loses some of their potential dps by leading, even Method's leader last Tier. But, if you're good enough you won't lose much and still remain near the potential. But I generally can't believe you can't push for more even if it's only a little more because for instance to use Ice Floes on an Arcane Mage proficiently and still talk and think on complex strategy at the same time is very hard to impossible.
I would say a healer has quite a good overview over the raid, what happens to people etc. During earlier stages of an expansion you have some more downtime, allowing for fairly easy raid leading.
Personally havent tried tanking, so I can't really say much about it, but sounds fairly decent to raid lead as.
Some dps classes have quite simple rotations where you only have to focus on the rotation and not too much on proccs, (perhaps only a 2 min use trinket and 2 min icd proc trinket) which makes it simple to raidlead.
This is a good point; not to trivialize tanking as a role in general, but it seems that in a lot of encounters the most important things to know as a tank are when to swap and when to use active mitigation. Of course, easier said than done and all that jazz, but it seems like other roles (ranged in particular) have to know a lot more about positioning and whatnot - when to spread, when to stack, kiting adds and bad shit, etc.
From a vantage point it's definitely the healer's advantage to lead though, you have a much better view of the encounter and also you know who stands in bad shit - primarily because if you don't heal their stupid ass, they're going to die
Tanking is generally an "easier, but you will wipe the raid if you fail enough" role. For instance they have to switch something during the encounter (taunt, move, etc.) and if it fails once or max twice it may mean a definite wipe. Imagine that if you had that on the DPSes the wipes would be a lot more since while their responsibilities are a lot more, they usually do not wipe the raid, and in many cases they may not even die on mistakes.
I would say that tank is the best role BUT in every guild I've been in the RL which were the tank have always had help from the other officers, especially one of the healers.
Tank would be the easiest because the tank is also the one that sorts of controls the fight in terms of positioning, add placements etc.
Would still be ideal if he has help from say, a range DPS/Healer to assist in times where he's blocked by the boss' groin.
As someone who's led a few raids as a healer, it's only worth it if you have voice chat. So much going on, it's near impossible to keep track of everything and keep people alive. Just my 2c.