FW that's the most important player. A top scorer is going to... well, score your goals
GK he comes next, after all you need someone to stop those shots from rocking your own net. But I am not sure if the GK is more important than getting another top FW.
2nd FW, is important almost or exactly like your GK. Why? simply because your 1st FW can't play on each match and besides, even when he plays he might have a bad mood or get jinxed. That's why he needs help up front.
DF more vital than MF on most occasions. You must have DFs to support your FWs and sometimes your MFs too. and they contribute to the GP and naturally DEF ratings.
MF vital only for ball control teams.
WG, that's a tough one. Can contribute a lot of the OFF, but so can a FW. My view is that WGs are best used when they are 15-19 SL. That way they don't steal shost from your FW. But if you get a player up to 25-30+ SL, why not make him a FW? One big bonus is that WGs are ignored for the 1:2 area ratings formula.
SW, I don't use them!
But should one choose do have one around,

make sure he is a real good player with 25+ even 30 SL. On Sesl, most managers go for the long ball or early cross approach, that means the SW gets 50% SL bonus for the DEF ratings. Can come handy sometimes. the downsides are that the chances of the opposition getting caught offside are reduced immensely, also the SW do not help to support the MF and FW areas.