When you're a JW, you really don't have much to look forward to so you're right, they attach a fair amount of importance to these silly jobs.
The order here is what I think
Parking - usually assigned to young males, some who aren't old enough to have their drivers license yet. Level of importance - low
Attendant - assigned to MS or if not enough MS avaliable then to non-elders who aren't on any "restrictions". Level of importance - low
Baker of unleaved crackers (regional, around here they buy Matzohs) - Places I've been don't even assign this, the speaker is responsible. If they do assign it, noone knows who purchased it. Especially since it could be handled by a female. Therefore, level of importance - below low.
Buyer of vino - see the cracker comment
Provider of table service - Ah, what a privlege to be a waiter!! Usually assigned to elders (maybe MS if not enough). Get to be seen by everyone, therefore the level of importance goes up to medium.
Prayer over bread - Always an elder. Importance - above waiter but below wine.
Prayer over wine - Always an elder. Importance - 2nd most (in my opinion). Bonus points if you can stretch the prayer out over 5 minutes.
Chairman (it's an official job starting this year, per letter to BOE) - Always an elder. Now mandatory, huh? Well, that moves up in importance a little I'd say more important than a waiter but less than a prayer.
Speaker - Most important. I've seen absolute fights over who gets this assignment. I've also seen lots of politics over it.
Buddy of Bethelite who pulled strings to get Bethel speaker to fly in - Priceless!