I haven't done one of these in a little while. Pantheon's Twitter account asked whether we like more, to play solo or play as a group.
I feel inclined to point out that this does not mean one exists without the other, merely a personal inclination toward one. You could even say you prefer a 70-30 or 51-49 split. They aren't mutually exclusive.
My personal tastes lean heavily toward grouping. Probably 90% grouping prefered.
I loved how people naturally drifted toward grouping in EverQuest. I like how UO/Darkfall/Sandboxy games encouraged communities of players to come together for survival. Most recently, my time in WoW (which has now gone on for over 6 months) has had its highs and lows -- the best times are spent playing with friends in dungeons, and the lows are having no one else log in and I'm forced to quest solo or find a group of people I don't know.
A game doesn't have to force one or the other, either. Sufficiently rewarding both activities works for me. I'm coming around to a more centrist position on the issue. Back in the 1999-2010 era I was very, very much about grouping and every game being a group-only game. Not so much these days.