With regard to established Trek ship design, especially in the TOS/TMP era, I try to evaluate a design as fairly as possible with regard to the design itself, not to any classical sense of "canon" design (and gents, it is "canon" when we're talking about Trek lore, a "cannon" is a big gun).
Anyone seen the FASA Andor-class "missile cruiser"? It's a piece of crap. The nacelles simply cannot be jettisoned without destroying the ship, and the secondary hull is a useless conglomeration of lines.
However, the Abbe-class photon destroyer was actually thought out. A standard saucer with a streamlined pylons linking to the nacelles, and the photon pod itself is self-contained and apparently has it's own reactors for the tubes. That pod could easily be replaced by a scientific package, cargo container, or maybe even a small carrier pod. It makes the ship itself very versatile in true Starfleet style, and is a very early version of TNG's Nebula-class.
The Chandley class that I've heard every-FASA-one loves is almost there, but the too-long warp nacelle pylons completely spoil the lines of the ship, as well as making them very vulnerable to enemy fire and even collisions.Now, I'm not overly sure where the nacelles should be on that ship, but the hullform of the bulky rear hull makes another location difficult.
I like ships that make sense and aren't just a re-arrangement/slight modification of an existing design. The design has to make sense to me. The Andor sure doesn't. The Chandley works, but isn't pretty or well-balanced. The Abbe is well-designed.
'Nuff said. *nods decisively*