I voted Oldman.
A guy who probably doesn’t get considered for lists like this because he does a lot of broad comic roles, but who I think is actually an excellent actor:John C. Reilly.
Glengarry Glen Ross has a few of my favorite actors: Kevin Spacey, Al Pacino, Jack Lemmon and Ed Harris. I also like Gary Oldman and Jeremy Irons.
…and deader than a door nail.
(Not eligible. See OP.)
I’m a huge fan of Jason Isaacs. There’s nothing this man can’t do. His range is enormous and his performances are always layered with nuance.
Jim Carrey? Steve Martin?
This is a very broad definition of “of our time:” some of these guys are in their 80s and DiCaprio is in his late 30s, and that has to be at least a generation or two in film history. I recognize the OP is asking in the sense of actors who are working now or have been active until recently, so that nitpick aside, the actors I am most impressed by these days are Bridges and Cranston. I know that in recent years Jeff Bridges has played a fair number of chill Jeff-Bridges type guys (and I haven’t seen his older stuff), but it’s impossible not to enjoy watching him do it. We’ve been watching Breaking Bad over the last year, and he’s excellent - and when you compare it to Malcolm in the Middle (a show I’ve seen but never liked much) it is just astounding that dimwitted Hal and angry, scheming, nihilistic Walt are played by the same person.
Jim Carrey had crossed my mind, he’s proven he can act, but I’m not sure if he’s in the same league as the others. Steve Martin as well. Yet, I don’t know if it’s just my perception, or relatively lowered expectations knowing their start was in comedy, but then, Tom Hanks has proven to be an exception to this thinking.
Jim Carry was fantastic in plenty of more dramatic roles, like The Truman Show, Eternal Sunshine, Man on the Moon, and I Love You Phillip Morris.
I’d put Jim over Steve in the actual skill and range of acting, though I love both of them dearly for different reasons, and respect Steve Martin more as a comedian, actor, writer, author and banjo player more than Carrey.
But Carrey does crack my shit up.
Still, I’m thinking of everyone still working today, DDL and Robert Duvall are hard to top (Eastwood, Oldman, Hanks and Hoffman not being far behind, for my own tastes).
“…There, you see now, how all your so-called power counts for absolutely nothing now, how your entire empire can come crashing down because of one… little… smack’s Oldman’s back …cherry.”
I loved that movie ever since I realized it’s a comedy.
+Lost in Space 
He totally sounds like Dustin Hoffman when he’s doing an American accent, and now I can’t un-hear it.
I haven’t seen him in much more than Awake though, which he was fantastic in.
This is tripping me up as well. I would argue that Robert DeNiro was the best male actor of HIS time which spanned several years and included breathtaking performances in Taxi Driver, Raging Bull, Cape Fear, Midnight Run, but wound down somewhere around Rocky and Bullwinkle.
What I’ve bolded is my yardstick for great acting, and I think Oldman, and Philip Seymour Hoffman do that for me. Also, I believe that the three men you mention in your first paragraph used to do that. (DeNiro and Pacino moreso than Nicholson)
I do get the distinction between best and favorite. I will see anything Alan Rickman is in. I think he’s a tremendous actor, in my top ten for sure, but not the BEST.
Hmm. I only voted for DDL, but I could actually see Leo DiCaprio doing all of those roles pretty damn well at various points in his career.
Oldman is very good, no doubt, but I don’t think he has enough significant roles for the “Best Of Our Time” title. I can’t think of a single role of his as meaty as Daniel Planview, but am willing to be corrected.
Depp isn’t getting a lot of mentions and I kind of want to change my oldman vote just to get him some love. He isn’t quite the chameleon that oldman is, and errs to the kooky too often but…
I too think Johnny Depp should get more recognition here. He is truly a chameleon on screen, performances have many terrific nuances, every character unique, never playing “himself”. I think Depp truly disappears into his roles.
I had to give a tie to Gary Oldman and Denzel Washington and Alec Baldwin.
I voted for 4 people, but if I had to vote for just one, Gary Oldman wins, hands down. The man is just phenomenal. He seems able to handle playing any type of role, and he’s also capable of being a supporting actor. The only person I’ve ever seen disappear into a role like Oldman is a guy who isn’t on the list: Harry Dean Stanton.
Before I hit “post” I scrolled thru the thread, and I gotta say that video that Spoke linked is an awesome overview of Oldman’s work.
I also voted for Gary Oldman, though it was a tough decision between him and Ralph Fiennes, who, surprisingly, has not been mentioned.
Schindler’s List
Quiz Show
Oscar and Lucinda
Spider
The English Patient
In Bruges
Coriolanus
Sunshine
Red Dragon
and, of course, Harry Potter
I’ll be interested in seeing him in the new Bond flick, because it looks like he’ll have to leave the brooding at home and try something a bit more approachable as a character.
I picked all of them. Because I could. 
Not sure I’d call some of these “our time” either, eg Redford. He’s done little acting in recent years.
As I’ve said, I’m on the Oldman bandwagon, but I’ll tell you another actor who’s not on the list and who I think is seriously underappreciated (possibly because of his pretty-boy looks): Colin Farrell
Watch these movies:
In Bruges
The Way Back
Hart’s War
Crazy Heart
Phone Booth
…and you’ll realize that the man can turn in a beautifully nuanced performance in a wide range of roles.
There is only ONE…Bruce Campbell!