Official DVD release - Limited Edition - might be expensive to come by. Look on ebay, amazon or country-specific e-shops. This release suffers from washed out colors and low resolution, looking more like a VHS release than a DVD.
Offical VHS release - There were many VHS releases but the best one is “The Faces” box set - a box set with face of Yoda, Vader and stormtrooper on the front. Obviously looks like shit since it’s a ~30y old technology. Again, look around ebay (some e-shops might have it but probably not).
Official Laserdisc release - Probably the best looking official release, depends on the state of the laserdisc you get. As previously, look on ebay (some e-shops might have it but probably not).
Harmy’s Despecialized Edition - The best looking version you can get. Created by a member of this forum, Petr Harmy, it combines footage from the special editions, film scans and the 2006 DVD to create a high-quality version of the entire Original Trilogy. The movies are in 720p with a very high bitrate, resulting in astounding quality - IMHO better than the official Blu-Ray, which had more than twice as many pixels to work with. The current versions are 2.7 for A New Hope, 2.0 for The Empire Strikes Back and 2.5 for Return of the Jedi. These can be found at various torrent trackers, such as KickassTorrents or ThePirateBay, but you may only find low-quality encodes, gigantic and for many people impractical MKV files or outdated versions. The best place to get the Despec. Edition is on MySpleen, which is a private torrent tracker for which you need an invite, but to get that you need to cement your position as a good user. Of course, it is illegal to download Despecialized Edition, so before you go look for it, make sure you own the official Blu-Rays.