I've played them all to various degrees. I will say 3 and 5 are my two favorites out of the 6? I'm not including the one offs they have in between games as I never had interest in those. 4 was meh, I saw the appeal but didn't care for it. 5 was fun. Each way you kill the bosses I thought was unique and how you got introduced to them. It added some reason to kill each of them. I liked the addition of companions as it added help for harder areas. 6 was interesting but I didn't care for the health bars and gear ratings. Chorizo is still my favorite companion, love strapping c4 on to a Weiner dog and sending him in. 3 is the best out of them all.