I think the Manhatten is the world's greatest cocktail. I prefer it with rye instead of bourbon (and definitely not Canadian Whisky like people in Wisconsin think), and absolutely with bitters. At the restaurant where I work, we offer a great version with Templeton Rye, Carpano Antica Vermouth (Vermouth that costs us $30 a bottle? Yes, and it's worth every penny), and bitters. At home, I've been using Rittenhouse Rye, which is outrageously cheap ($14 a bottle!) and the regular Carpano Rosso, along with the classic Angostura.
As for the martini, I definitely prefer the original gin to vodka, and like Hendrick's, Beefeater and Magellan as my gins of choice. I usually do a lemon twist instead of olives. As an aside, martinis without vermouth are not martinis, they are glasses of cold, watered-down gin (or vodka if you must). Which is fine, in and of itself—I like Stoli straight up on occasion—but not a martini.
I'm also a big fan of sidecars and daiquiris (not the frozen kind). It's fun to experiment too. A golden rum sidecar is a thing of beauty I've found, and when I get another bottle of cognac (hasn't been on sale for a while) I'll try a cognac daiquiri.
Oh, and I almost forgot the negroni. With a slice of orange it's great for a hot summer day.