Let's see:
Campaign where you don't expect to ever level up (RP-heavy games, 1-shots, lots of PBP games, some RL games where the DM is stingy with experience:
Vampire! Start as a member of the living, and it's relatively easy to become one - make a deal with it and have a contingency plan to avoid becoming its mind-slave. Instant +5 LA template.
Lich! If you're starting at 17+ as a wizard (or maybe cleric), I'd just prepare the materials for lich and start the game by transforming into one after an adventure or two. Have a high-level minion with a non-magical amulet as a decoy phylactery for added style.
Campaign where you do expect to level up:
Grave-touched Ghoul from LM is a pretty solid template to make you a ghoul - much better than the SS levels. +2 LA.
Necropolitan is good in many cases, especially mid-level games.
A late-bloomer, but I'm a fan of 10 levels of Walker in the Waste (Sandstorm), which turns you into a lich without the LA (presumably without the LA, otherwise the PrC is complete rot - but other transformative PrCs like Dragon Disciple don't have LAs).
Niche cases:
For the right build, a Ghost (+5 LA) can be a lot of fun. Good for scouting, Mind Jar-ing NPCs, skirmishing, etc.
Protecting yourself from the living:
The first thing you should probably do is establish a gentleman's agreement with the DM. Point out that it's relatively easy for higher level NPC clerics to utterly destroy you, without you even getting a saving throw. Point out that save-or-die's-without-a-save really, really suck, and add in that it's probably also easy for you to create a horde of shadows, vampires, and wights and take over the world. Ask if maybe you two can agree not to use either tactic.
Second, consider checking LM for spells and feats that protect you from turning. 1-2 items and maybe 1 feat should be enough. Honestly, I wouldn't even bother most of the time - if the DM's going to throw clerics at you consistently, you've already lost the fight, and if he only throws one or two over 20 levels, then I'd rather have that feat and gold spent on something useful for the other 500 fights you had earlier.