I'm pleased people are bringing up games from the Simple 2000/3000 series. I think they're pretty much the definition of "awful but can be lots of fun". Earth Defence Force was born out of Simple 2000 and it's actually become quite a success.
Anyway, my contribution:

Slow, clunky, squad based shooter with a horrendous story. Yet I've completed it about 3 times and keep going back to it every now and again. Although I realise the gameplay is awful I actually have a lot of fun with it,







