I used to love Yu-Gi-Oh and Pokémon cards as a kid, but the former is too complex now, while the latter is literally unplayable without enough energies. I don't know about these days, but when I was a kid: Pokémon booster-packs where really stingy with the energies... So I mostly just collected them.
I don't really play playing-card games, so I can't take those into consideration. I like Uno and Phase 10, and still play them occasionally, but I'm unsure which I like more — but my favorite card game would have to be one of them.
They're pretty simple and you only have to buy one pack to play, which are pluses both games share. The reverse cards in Uno can be quite confusing, but they do add variety to the game. Phase 10 can last forever, but there ain't no'n forcing you to play every round.
What's your favorite card game?