Beast 1 was one of the games I got very early when I had just got my A-500.
I liked it a lot and I considered it the best Amiga game at this time. Only problem was that annoying things where it was just luck how much energy you lost.
Then I got Beast II and was impressed by the grafix, but soon found out that the game was unplayable because of it's difficulty level. So for a guy like me who never cheats it was a worthless game.
Beast III on the other hand was very playable ! Also a big advantage was the password system so that you didn't have to start again all over from from the beginning
I finally completed Beast III and I remember I found it a great game and was a little bit sad that I had finished it in a rather short time.
So, I would say Beast III is best in it's playability and maybe grafix also, but Beast I was impressive when it came out in '89 !