I've tried quite a few; none of them were easypeasey to set up and get working faultlessly.
Serviio was the best DLNA server I found. Installs to your current distribution; got quite close to out-of-the-box streaming to my Sony Bravia TV. Took quite a bit of fiddling to work perfectly; that involved downloading and compiling ffmpeg and other software. It also streamed nicely to other DLNA devices about the house. I was running it on a P4 2.8 ghz and it handled the transcoding of all the media I gave it. (Playback only being funny with DVDs I'd ripped myself; turns out I had to fix the framerate to PAL)
Openelec was the best of the 'plug-it-in-to-the-tv' types. Based on XBMC it installed and worked out of the box onto a Rasberry PI. Although laggy sometimes and unable to really handle anything other than the default skin. Had the added bonus of decoding the TV remote signals via the HDMI lead via some magic.
I'm currently running this via a P4 2.8ghz attached to the TV via VGA using an old windows media player IR remote. Almost worked out of the box. Streaming had no sound; which took some fiddling to get to work (Although Apples airplay still has no sound...)
Openelec and XBMC both have the feature that they will only show you films nicely (in that cool coverflow/fanart interface) if you keep your media tagged and correctly organised. (See documentation for what XBMC thinks is correctly organised.) If you have a big collection of movies and tv shows with somewhat hard to interpret names; be prepared to spend some fun time tagging an organising them. (I recommend tinymediamanager to handle that job)