I'm personally of the opinion the ROI on spending coins, the other form of in-game currency in the game which can't be purchased, to buy packs is too low to make it worthwhile. If you're not willing to buy FIFA points to open packs, your best bet is to use the coins you accrue from playing games to buy players on the transfer market.
If you're going to spend money at all, the Mega Packs (aka 35K pack), Rare Players Packs (aka 50K packs) and Jumbo Rare Players Packs (aka 100k packs) give you the highest odds of pulling the best possible players. They are not coincidentally the three most expensive packs you can buy.
Can I get one of those super charged cards just by playing games?
Maybe. EA in the past have done special tournaments which if you win (either against the computer or against fellow FIFA players) result in packs (or rarely a player) as prizes. The packs in these prizes are usually untradeable meaning whatever cards you get will either be part of your team forever or you'll have to discard them for no coins.
How does Team of the Season (TOTS) work exactly?
Each week, one or more team of 18-23 players representing the best from a particular league or geographic region will be released into packs. When those special cards are in packs, they replace the ordinary (or non-in form) version of those player cards. When Liga BBVA, the biggest and best league in Spain, has their TOTS in packs, for example, you won't be able to pack pull the ordinary 90-rated Neymar. Instead, only his blue higher rated card will be available in packs.