The tool that I feel most relaxed after using is the lathe. It isn't a real practical tool in terms of making things I need, but it provides a great deal of enjoyment. Quick turnaround projects, nice little bauble that people are just amazed you can make. Last night I turned my first hollow-form, about 6" tall by 3.5 wide, neck opening about 3/4". From a piece of scrap mahogany deck post (free, another nice thing about the lathe, uses lots of free wood).
Everything went well, too well, I got greedy on wall thickness. Had a minor catch and my reaction blew the vase apart. I had got the walls down to just under 1/16" thick. My kids (9 & under) still wanted to keep. I put tung oil on it, gorgeous except for that shattered rim. It will remain on a shelf near the lathe as a reminder to not get greedy.
In order to turn things you have to completely concentrate on tool position, body position, everything. But not only from a "how do I keep my body parts intact" point of view like most tools (you've got that with the lathe too). It's focusing on the shape, the way the shavings come off, how the cutting is going. Completely different feel than using most power tools, which is more of a feed the piece to the machine feel.
In terms of which tools do I most dislike using it's probably the jointer. Too many nasty stories, the somewhat exposed cutterhead. Just waiting to gnaw off fingers.