I'm a big fan of ST I-IV. I've haven't gotten every ending with every game, but I've seen at least half of them.
Part III was by far the best. Its one of my favorite games of all time. Part IV was, like SuperDeadite said, the ending of the series at that time. Honestly, the thing feels like the end of Sega as we knew it up until then too. Yoneda has some lines at the end that nearly brought me to tears. Ostensibly he's basically just saying, "Live youth while you have it, because some day you'll be over the hill. I know." but to me, and I think a lot of Sega fans, it seems like he's saying, "We at Sega did our damndest, but as soon as this game ends you'll never see us again. Thanks for playing." The fact that STIV was even made at all was quite a surprise, further cementing it as an intentional farewell from whoever cool was still left at the company at that time. In the decade since STIV came out you can count the number of good Sega games that have been released on one hand. During the DC days they were nearly a monthly a occurrence.
Both part Vs just looked like crap to me, much like new Sonic games. I haven't played them though.