I'm partial to STIII myself (since it also happens to be the first Trek movie I saw in a theater), though I'd still say it's more victim than conqueror of the third movie curse since STII was arguably a better movie. Sure, it has some great moments and concepts (the theft and subsequent sacrifice of the Enterprise, McCoy carrying Spock's persona, the Promethean aspect of Project Genesis finally coming into play), but STIII is still lacking in a lot of places, too, and it shows - even at its best, it's really little more than the dot connector between STII and STIV, must as BTTFII is to Parts I and III.
And it's for that same reason that I've come to prefer BTTF Part III over Part II, all the way up to the end - I don't give a damn how silly everybody else thinks the flying train is. Honestly, was it GOING to come anywhere close to Part I? Were either of them? Zemeckis and Gale have maintained they never had any intention of making any sequels when they made the first one, and watching Parts II and III, I'm actually inclined to agree (Part II even more so).