lol
EDIT: okay I've decided to add a bit to this post.
1 - Tighthead.
The master piece in the scrum. You get penalties from scrum, that's 3pointers, a platform to operate from, penalty tries occasionally, a strong choice to pick when you've got the penalty, a way to intimidate and set the tone mentally, ascendency in the pack for 80min. Takes years to develop a good TH, they're only really good during a fairly narrow window of time, they're the rarest commodity.
2 - Flyhalf.
Sexton, Wilko, Quade Cooper, Dan Carter, Trinh-Duc, Nico Sanchez: they dictate the tempo, sometimes the shape of the game and they influence it tremendously. Not a whole lot is more important than that.
Just one example: France are on top of the world during the 2012 EOYT because Michalak was the best 10 around during that month. He starts playing like shiit to start 2013, the whole team is subjected to a downward spiral.
And of course, classically your penalty boot. Most of your points will come from him. And points are vital.
3 - Flanker
I won't separate BS and OS because it's not always clear-cut, esp. for me being French, but so much of the game is won at the breakdown, today more than ever, and look at the ERC player of last year, it was Steff Armitage because he was a bdown masterclass. You win the ground, you win the game. Bdown is the new scrum in a particular sort of way. Tackling with high workrate, fetching, slowing down rucks, clearing out, carrying hard and heavy, lineout catchers, lengthy, powerful, athletic...very important.
4 - Scrumhalf
box kicking, insuring the transfer from rucks to FH and backline, general playmaking, being the crafty little brainy wiz. Very important.
5 - Hooker
I don't have an example of a team really missing smt when they only had a mediocre hooker. As long as he can throw a bit for lineouts, hold up decently in scrums, and offer extra hands in the loose...but still a very specialized, technical position that requires experience and dexterity that don't just appear magically.
6 - Fullback
last line of defense, kicks a lot, must perform under pressure constantly, under high ball, insure continuity during counters, and is counted on to offer an attacking dimension and beat defenders off the step...very important.
The rest: centers, wingers, LH prop, no.8, locks...every position is important, but I feel they're less *vital* than the above.
- Wingers: you'll see teams with very average wingers who still score tries because they mount a play patiently and the offense will come. Plus, you don't have to score a ton of tries to win games. England is the classic example, more recently their 2013 campaign. A top team, sterile offensively.
- Centers: there are tons of average/okay centers in the Top 14 (which I know best) who get the job done well. They're not all 105kg who step, beat defenders, punch through the defense and make clean breaks, are Conrad Smiths with the passing; but they're enough to insure continuity for the ball and offer sound defense. You don't *need* great centers to win.
- Locks: counted on for the work, not the skill or talent. They're warriors, and chances are if they play Rugby they'll commit for 80min. Some are outstanding, of course, but a good solid 1m95 115kg lock who commits to the rucks, maybe catches LO throws, okay work rate, basic handles, is all you really *need*.
- Loosehead: you look at a Marcos Ayerza who's the stronger element of a scrum and he's practically assuming the role of the TH, completely neutralizes/incapacitates the opposing TH, sure a LH is important for a scrum. But they're not the most important in the loose, aren't often great defenders, and generally I wouldn't put them in the *vital* column.
- No.8: probably my favorite position, but just not nearly vital. Yes it's cool to have a Louis Picamoles, operates from the base of a scrum inside your 22 and takes off to punch in a try on 3 of your defenders, carries hard, is a platform on his own, good under the high ball, serves as a fourth back three often...but this is all added value, an extra weapon. Not nearly vital.