Personally I prefer the different kinds of flavored coffee creamers you can buy (like International Delight) over the syrups. Plus they come in some "diet" forms if people are concerned about that type of thing. I think you'd be able to buy more varieties of those than the syrups for the same money. I guess that doesn't exactly answer your question! If you are going for the syrup, I'd get something like caramel, peppermint, vanilla, and maybe a sugar-free option. For recipes, they have a helpful search section right on their site: http://www.torani.com/home/recipes

I use Monin syrups, but our favorites are caramel, vanilla, gingerbread, peppermint, and raspberry. We have seen the raspberry curdle the milk tho. None of the sugar-free variants I have had worth worth buying.

I stay away from the flavored creamers I have seen because they seem to have overly processed ingredients.

a website that I am not affiliated with that I like to buy syrups from is Whole Latte Love, and you can find them with a search engine if you care to.

I love Torani Syrups. Vanilla, Caramel, Hazelnut and Irish Cream are the common coffee flavors. Be sure to have a sugar free option (they make those) I get these variety packs (gifts with coffee or protein - I do shakes a lot) http://theworldaccordingtoeggface.blogspot.com/2011/11/heads-up-sugar-free-torani-holiday-pack.html a few months ago they had a holiday assortment too. I like it better than creamers as they are often full of oils, fats, some are dairy, and they often require a fridge or ice.

I think almond and caramel are good solid ones. A really delicious and fun one is toasted marshmallow. Another good idea would be to offer (real--not syrup) cinnamon, cocoa or chocolate shavings, whipped cream, etc. Very inexpensive and also a great way to add additional flavor to the coffee. As for other ideas with the syrups, you can make flavored milk shakes with them, pour them over shaved ice, use them over ice cream, drizzle over fruit, use in cocktails (a dash of any syrup combined with champagne is a fun and easy cocktail). You can also use them to make delicious and unique homemade sodas (just combine seltzer + syrup of your choice). Those are just a few ideas I can think of!

I love the Salted Caramel and Peppermint Torani Syrups, they offer a nice change in my daily coffee addiction.

I'm addicted to the Torani Sugar Free Peppermint syrup- I drink it in my coffee year round!

Definitely the basic vanilla and caramel, and one of those two flavors in sugar-free. I also really like amaretto.

I love the Sugar-Free French Vanilla and Irish Cream.

Hazlenut is the popular nut syrup. I'd also bring at minimum:

Vanilla,
Caramel,
Sugar-free Vanilla
2 more fun flavors like Irish Cream or Chocolate Mint...

Whatever you do, don't get any with red dyes! I used to work at a coffee shop and the red ones curdled milk/cream basically on contact!

Starbucks also sells liter sizes of their syrups; you just have to ask. I love toffee nut from there, but as far as Torani syrups, I'd have to echo everyone else: vanilla, caramel, hazelnut, peppermint.
I'm also in 100% agreement with chocolate shavings or cocoa and cinnamon to sprinkle on top.
Also unsweetened, unflavored cream/half and half/milk is a MUST.

The syrups can also be used to make Italian sodas, you could use it to flavor whipping creams, any hard cocktail that you would otherwise use a simple syrup for, flavoring milk shakes... There are so many options!

I have worked at several coffee shops, and I think you definitely need vanilla, sugar-free vanilla, and caramel. I think the next option, seeing as you already have hazelnut, would be some kind of chocolate (sauce, not syrup). A personal favorite is amaretto, but peppermint is more popular this time of year. I was always surprised at how unpopular typical ones like Irish cream seemed to be.
The trick with the fruit flavors that others are commenting on, probably too involved for your situation, is to mix the syrup with a cold dairy product before adding it to the hot coffee, not trying to mix it all at once. It is kind of a pain to make a drink that's actually good that way, in my opinion.

Personally I've tried a bunch of flavors, but don't enjoy any of them. Make sure you have plain sweetener for folks like me! But that being said, I work at a coffee shop and will say that Vanilla, SF Vanilla, Irish Cream, and Almond are the most requested.And as many have mentioned above, they can be used for so much more than just coffee flavoring so experiment!

My boyfriend loves the Toffeenut syrup at Starbucks in his coffee, and we found that the Torani Almond Roca is a great substitute. We both use it every day.

I'll second the Almond Roca--it's delicious. Also, Monin apricot syrup is surprisingly good in coffee, but it might not be the best choice for a large group.

Kind of abnormal, so maybe not good for an office party, but maple syrup is a great alternative to the traditional flavoring syrups. It has a really wintery-autumnal taste that is similar to Starbuck's gingerbread or or spice flavors.

What? No Torani Bacon comments? And no, I'm not joking. I've been tempted, but have yet to try it yet.
For the ones I actually like, I agree with Vanilla and Hazlenut and second Almond Roca. I personally can't stand the SF, and when I worked at a coffeehouse, I wouldn't recommend them unless you were diabetic.
Cost Plus World Market generally has good deals on Torani (better than Starbucks).

I like the caramel, hazelnut, and french vanilla ones (but I was buy sugar free).

This entire post and thread makes my taste buds hurt. Why would anyone want to have a coffee-and-syrup party? Gross!

Once you get the syrups, it's very handy to have the pump-type dispensers that coffee shops use. Amazon sells them, but they're hideously overpriced. Here's one way to get them for free or cheap:

Go to a starbucks, and ask to buy some syrup. Ask if they can throw in a pump dispenser. Mine gave me one for free, and probably would have thrown in a couple more if I had asked.
BTW, I prefer vanilla.