The Room is a classic that I've loved since the very first time I saw it. I've watched it several times now and it never gets old.
Not sure if it counts, since it is intentionally bad, but Sharknado was great when it was first a thing. Unfortunately, oversaturation of the shark film market has diminished this film's overall quality, which is a shame because I loved it before shark films became mega popular.
Super Mario Bros. This movie is actually painful to watch. However, I still find myself enjoying it for some reason. It's definitely not a so-bad-it's-good film like The Room, as it's just plain bad, but there's something about it that somehow stops me from hating it. It's a weird situation that I cannot really explain.
Food Fight. How do I explain this movie... it was an attempt to adapt brand names like Mr. Clean into a life setting, but as it failed to obtain the rights to many brands and needed to make up replacements, it fails on many levels. The animation and models are also horrendous and the voice acting leaves much to be desired, but I found myself unironically enjoying the film. If you haven't seen it, I suggest you check it out. I doubt many people will feel the same way about it as I do, but I still encourage you to watch it for the lulz.