i used to play with several friends, but stopped buying any cards once Tempest came out (i thought they were going a bit overkill on a few of the cards... part of Weatherlight was the same way)..
I never really got into playing online, because I found most of my friends whom I played online would almost always had 4 mox's in their decks and a few black lotuses.. I personally liked having to collect them and if you had a rare card it made it that much cooler.. Most of my friends didn't drop too much cash on decks, so I had never seen a mox or a black lotus played in any games I was in.
since there also werent any gamestores around me who held magic tournaments (until after I decided to quit buying cards... bah Tempest) I never did anything other than friendly matches.
Now, sevferal years down the line, a friend of mine's little brother started to play so I basically gave him everything I had that wasn't currently in a deck so my collection was easily cut down to a 1/5 of its original size, but I rarely ever miss them (only miss them when trying to help a new player play)..
I am planning on starting to play agani with some of my older decks against some people in town here (different town, different folks, different playing strategies maybe?) and see how my decks hold up since most of them are only revised with a few cards splattering up to Weatherlight (although few weatherlight cards)..
as far as my favorite card to build a deck around.. hmm tough call... I would say functionally I like the Vesuvan doppelganger, but since I absolutely love the picture on Clone I will go with Clone
(disclaimer: please forgive any grammatical errors in above text... tis late, and I did notice a few, but I am too tired to reread the entirety of this text)