Currently I'm going with, mostly for range-illustrative purposes and attempts to prove that he does not play the same character in every single film:

Bill & Ted's Excellent Adventure
Constantine
Speed
The Watcher
I Love You to Death
The Devil's Advocate
Under the Influence
The Gift
River's Edge
Hardball
Thumbsucker
Freaked
A Scanner Darkly

...and I should stop because the list is getting long. Hopefully by that point they would either be a fan or a well-educated fish.

and MOPI. Definitely. ;)

and as a treat I'd offer them 'Street Kings'. by that point they'd either be able to see the subtleties of K's acting, or to at least get the Ludlow jokes on the IMDb forums.

Thumbsucker
A Scanner Darkly
The Gift
Devils Advocate
Bill & Ted (1 & 2)
Street Kings

Speed
Constantine
Street Kings
The Matrix
Matrix Reloaded
Matrix Revolutions
A Walk in the Clouds
The Replacements
Sweet November
The Lake House
Point Break
The Day the Earth Stood Still

I haven't seen MOPI yet, and there are several others I've still not seen. Bill and Ted I remember parts of.

1.) My Own Private Idaho
It's a difficult choice to make and it might not be the ideal film to start with, but it was the first film with Keanu I could actually remember. It's also not the most accessible film, but his acting was great in it.
2.) Constantine
Anyone who wants to see how well Keanu can act MUST see this film. I was amazed. It might even be his best performance. I totally bought his John Constantine. He breathed it. He WAS John Constantine.
3.) River's Edge
Just as the reviews told me, this was a rather unusual teenage film with a twist. Keanu's Matt really touched me. It's best watched on a quiet day, where you can get into the mood of the film.
4.) A Scanner Darkly
This was the first Keanu film I have ever watched. It was confusing, I couldn't really get into it, but I still loved it. I love the combination of troubled human beings dealing with drug addiction in a corrupted world. Sadly, the film very much represents very real and very present and current circumstances. The film is very creepy, especially towards the end. It's sad and depressing. And Keanu pulled the character off so well. You could really see this troubled human being fighting with one of the cruelest situations in life. He seemed helpless.
5.) Point Break
I was positively surprised with this film. It had depth, it had elements of friendship, love, betrayal and believe. Johnny Utah seemed troubled and unable to integrate himself into neither his work as an FBI agent nor as the surfer-to-be, originally his cover, but more and more his "real" identity.
6.) Thumbsucker
Perry Lyman was hilarious. I think this film is entertaining enough for someone who does not know Keanu too well, so it is good enough to be watched towards the "beginning" of the list.
7.) Bill & Ted's Excellent Adventure
8.) Bill & Ted's Bogus Journey
Best watched together, as with many trilogies and sequels. I love Keanu's Ted. He's one of the most believable characters Keanu has ever played. Ted is so pure and honest and naive. Only a good actor can pull this off. Ironically, if you think about all people slagging him off. I think a lot of actors familiar with comedy (i.e. Michael J. Fox and the like) have to be really good to pull it off so well.
9.) Something's Gotta Give
Bbut only because I am a sucker for Dr cardiologist Keanu 'Julian Mercer' Reeves. I'm not a great fan of romantic comedies in general, but I loved the character. He had style.
10.)Young Again
Here I love the vivacity of Keanu's character Michael Riley. The film is terrible in most possible ways, but I loved Keanu's acting in it. It is very much like his own 20-year-old boyish self. It can be a fun film to watch because it's so cliché and yet has Keanu with all his heart.
11.) Johnny Mnemonic
Not everyone will like this film, but I like Johnny Mnemonic enough to recommend it to other people. There's always someone who won't appreciate it, but that's only normal. ^^
12.) Speed
I didn't like the film that much, but Keanu did rather well. Plus, I love Sandra Bullock, so that's two great things at once. Why would I tell seomeone else to check it out? Why not? Keanu's acting is well enough. He's a pretty good cop, even though I don't like cop films or any sort of action film.
13.) Parenthood
To be honest, even better than Keanu, in my opinion, was teeny weeny Joaquin, then only 15, awwwwww. He was awesome. As for Keanu, he did a decent job. He made me laugh in the film. And I love Todd Higgins' and Gary's relationship.

Films I wouldn't recommend:
16.) Dracula
14.) Chain Reaction
13.) The Day The Earth Stood Still
It's more about the films rather than Keanu's acting. Dracula is good enough if you want to watch a film adaptation of Bram Stoker's novel. It seemed very authentic. But Keanu's acting doesn't really figure in it, in my opinion. Chain Reaction would be the one film of the bunch where I'd say "OK, you may watch it, his acting is pretty cool, but the plot or the not-plot is too distracting and doesn't enable you to enjoy Keanu's acting. The Day The Earth Stood Still was just too blockbuster. Nuff said.