Wow, those are some very different games in regards to both play and theme. Let's break them down...

1. This is a dream game for many collectors, but a lot of people will argue that if you're just looking for something to play and enjoy that there are many more games that can give you equal (or better) enjoyment for much less money.

2. ToM is a very fun game and it looks fantastic. A common criticism is that it starts to feel repetitive over time, but I feel it's a game that's worth owning for as long as it takes to start feeling that way. It holds its value well too, so it's a good first machine because you can usually flip it for what you paid, possibly more if you take care of it or improve its quality.

3. 24 is good fun, but it's a little new to know how long its legs are going to be. If you want to try this one, your local distributor will probably have one on their showroom floor that you can try out. I've only played it briefly, and it seemed pretty cool. I'm not huge on the theme though.

4. Who Dunnit? is a game with a very unique flavor. There's one on location near me and I like playing it when I get the chance. It does feel like you're making the same shot quite often, but due to the interactivity of the story, it doesn't feel as repetitive as it maybe should. This is one game that visitors will be hooked on if they pay attention to what is going on up on the screen and are trying to solve the murders. This game, however, feels more complimentary in a collection to me than a good showcase game.

5. (you skipped 5!) Tales of the Arabian Knights is another solid game. This one has a fun theme, in my opinion, but it gets pretty mixed reactions. I do think the playfield looks great, and there is some pretty attractive ball movement. My biggest criticism of this game is that the flow can be broken up pretty abruptly by some drains that don't quite feel fair.

6. White Water is awesome! It's probably one of the cheaper priced games listed up there, and it's most likely the best "starter pin" for a collection. There is a A LOT going on with this machine, and it's sure to excite your visitors when they see the ball zooming around the neatly shaped ramps. This one can be a bear to tear down and shop, however, so keep that in mind.

Out of the games you mentioned, I would probably pick White Water because it's on the cheaper side and it's a very well designed machine. Who Dunnit? is probably going to cost less, but the gameplay is much better with White Water. The other machines you named are good, but they'll cost you twice (or more in the case of Cactus Canyon) than what you'll pay for a good White Water machine. Whatever you decide, make sure to come back and post pics!

Good luck, and don't rule out additional options. There are lots of great games out there for a first-time buyer.

I've been a Cactus Canyon owner for 8yrs and have not gotten tired of it. I was lucky to fall in love with it at a local bar and when the owner decided to sell off the business he offered it to me at a steal of $1200. That was a deal then, let alone now. If you dont mind spending the money for the cost it is worth it. Repairs have been minimal with only having to replace broken plastics when I purchased and replacing a rollover switch here and there. You will want to keep a couple GoldMine entry's around.Will retain value if you take care of it.
TOM is a great game, I can see where it can get old after awhile, but I never got to that point. I have a friend that has one she plays alot and loves. As for repair costs for that machine they have been very minimal, just have to keep replacing the trunk if you play alot. Will retain value if you take care of it.

As for the other machines... say no to 24 or any relatively new Stern game. When I come across them in the wild they often have something broken. They are just filled with plastic gadgets that give little challege to what a good game of pinball can be. I never seem to fall in love with most of thier games. They also look like machines that will cost alot to maintian in the future due to all the gadgetry and plastics.
Have not played Tales of Arabian Nights or WhoDunit enough to comment. Whitewater is ok in gameplay, but not something I would choose to own.
FYI I am a fan of 80's-90's Williams and Bally machines, they are relatively easy to fix and most of the design themes are origional. I also like the feel of EM games that rely more on your skills of shots and nudging to keep you entertained rather than gadgets.

Can I suggest you buy one of these computer simulation DVD's off somewhere like Ebay for a few dollars so you can try each game yourself to see which one keeps you coming back for more? That way when you spend your hard earned cash you won't be disappointed!