Bourbon and coke (or 7up). Short, simple and easy to mix. Same goes for rum.
Jello shots are a nice novelty and will fuck you up quick if you're not careful. Vodka + cherry/lemon/orange, tequila + lemon/lime are good combinations. Tip: marinate some diced fruit (pears, pineapple, peaches) in your liquor of choice and mix in. Flavored liquors like schnapps work well, too, but have less kick.
Martini's are easy to mix, but hard to mix right.
'Rita's are best served "on the rocks." Don't buy a bottled mix, though. Bite the bullet and squeeze your own limes. (Best done ahead of time. No one wants to be stuck in the kitchen squeezin' when the party's in full swing.) The effort is worth it. Don't skimp on the tequila and <insert orange liqueur here>, either.
Have fun.