Yeah Cassel is brilliant. Thought things were going a bit downhill for him after doing the Oceans 11 sequel but he was back on track in Eastern Promises and of course Mesrine, which surely takes over from La Haine as his defining part.
Steve Buscemi is another one who's always watchable. That said, he's normally in good films, it's not like he has to rescue them. How about Robert Downey Jr - I enjoyed the Iron Man films but without his performances it might have been an entirely different matter.

JD Walsh
Samuel L Jackson
Helen Mirren
Catherine Keener
Joan Cusack (oh indeed, and concur with your other selections)
Emma Thompson (Remains Of The Day onward)
Jack Lemmon
Gene Hackman (it was a rare film which matched him)
Kathy Bates
The entire Huston family
Tommy Lee Jones
Harvey Keitel
James Woods
James Mason
James Stewart
Henry Fonda
Lee Marvin
Warren Oates
Robert Ryan
Patrick Magee (getting further back now, better stop).

I'd like to include Richard Harris. But I can't.

I am always happy to watch William H. Macy in anything, he always seems to tick the 'everyman' box expertly.

The best around me thinks.

I'll second the vote for Gene Hackman - even though 90% of his films are cack, he's usually brilliant.

Nicolas Cage
Denzel Washington

I'd watch them read the phone book.

I mean Jack Nicholson, of course. He's great in everything.

I'll also watch any old shite as long as Ray Winstone is in it

Great voice and acts with a twinkle in his eye and a spring in his step.

Go and see her latest 'Partir' for a real acting masterclass.

No Ed Harris ? Agree with your choices,Paddy.
I'll watch anything with Harry Dean Stanton in it too.
Of Newer actors Paddy Considine is usually excellent.

What about Robin Williams. I miss Mork and Mindy...

Morgan Freeman - conveys so much style, charisma and gravitas in everything he does

Christopher Walken makes every film he appears in, just a little bit weird.

he won't be making many more movies. Nor Steve McQueen who was magnetic lying down with his eyes closed.
Johnny Depp too.

undoubtedly a great actor. but just watch a movie he did a few years ago called Satan, about a lot of Wicker Man style goings on in a French village. Absolutely awful. Cassell over acted his part by a country mile. gory, plotless and the biggest load of cack I've ever bought on DVD. the dvd is now in an oxfam shop where it belongs. He is much better in crime thrillers.

Bill Murray (because he never disguises the fact that he knows when he's in an awful film)
Javier Bardem
Frances McDormand
Jason Bateman (has been in some truly dreadful films,Couples Retreat for one, but I can forgive him anything)
Allison Janney
Joseph Gordon-Levitt
Guy Pearce(perhaps borne of too much Neighbours as a child)

That's probably enough for now.

Short but great

Godfather Parts 1 & 2
The Conversation(one of my fave fillums)
The Deer Hunter
Dog Day Afternoon

Paul Giamatti, fine character actor along with Buscemi

Paul Giamatti deffo - John Adams was superb.
Here's another - Stanley Tucci, a riveting actor.
Samantha Morton
Heather Graham always puts a smile on my face although quality control seems to be an issue now.
On the comedy front - Steve Carell and Will Ferrell are always funny even when the product sucks, although I know that last statement may be divisive among the massive.

John Cusack is never less than completely watchable.

Walken.
Samuel L. Jackson.
Harrison Ford.
Martin Sheen.
That really ugly French fellow from Amelie and Alien Resurrection.
Charles Dance.
Andy Garcia.
Gene Hackman.

Stephen Rea and Stanley Tucci

I agree about Stephen Rea. There is something about the fellow. He went out of his way to say hello to me for some reason, backstage at a mutual friend's do, but then someone else whisked him off - a pity, I'm sure he's an interesting chap and I'd have enjoyed a conversation...

Several of Alatair Sim's early film appearances were more or less one or two scenes and he often 'stole' the film on the basis of them - like his part as an eccentric (his parts were ALWAYS eccentrics!) comic book writer in 'Hue & Cry', the first Ealing comedy. Here he is in a self contained routine in Hitchcock's Stage Fright...

Michael Caine
Bob Hoskins
Ray Winstone

Will happily watch anything with either or all of the above in

Well, I’m standin’ in line in the rain to see a movie starring Gregory Peck
Yeah, but you know it’s not the one that I had in mind
He’s got a new one out now, I don’t even know what it’s about
But I’ll see him in anything so I’ll stand in line

Jeff Bridges. A lot of his movies know it too. They just bend to his excellence. How else can we explain the sheer number of beauties he's been in, probably more than any other screen actor?

And Tom Hanks for the last 20 years. The only top-line 'star' whose company I'm always happy to keep.

Daniel Auteuil
Tom Hardy
Judi Dench (though The Chronicles of Riddick was a low point)

whether in popcorn fodder like Transformers, indie cults like Big Lebowski or Barton Fink, or great films like Fearless, he is almost always the best thing in it.

by commenting on my own thread, I have a couple more because I am watching the Bukowski movie, Factotum, on BBC2.
Indie movie favourite Lili Taylor is always excellent as is Marisa Tomei.
I especially love Marisa in The Paper and My Cousin Vinny.

I have to say, I've seen almost everything Leonardo Di Caprio has done in the last 10 years and he's been consistently excellent. Can't actually think of a bad film he's been in recently though, so perhaps that would be the true test.

James Woods

Midnight Sting (awful), Any Given Sunday (adequate), plus a whole lotta duff, yet he's great everytime!

... never lets you down. Fine actor.

Oldies:

James Stewart
Katherine Hepburn
Jack Lemmon
Walter Matthau
John Mills
Richard Burton
Ronald Coleman
Robert Donat
William Powell
Dirk Bogarde
Bette Davis
Gene Kelly
James Cagney
Cary Grant
Myrna Loy
Stanley Baker
Elvis - doesn't matter how ropey the film I love Elvis in movies
John Wayne - never used to see the appeal until I watched Rio Bravo for the umpteenth time and marvelled at his unfussy manliness and gruff charm. Since then I've enjoyed his films immensely.

and the daddy for me:

Robert Mitchum

Current:

Gene Hackman
David Strathairn
Jack Nicholson
Gael García Bernal
Penelope Cruz (if it's a Spanish film!)
Donald Sutherland
Bill Murray
Daniel Auteil
Judy Dench
Emma Thompson
Audrey Tautou
Meryl Streep
Russell Crowe - not everyone's cup of tea but I do think he is a great movie actor
Mark Ruffalo
Laura Linney

And the daddy for me

Leonardo Di Caprio - he's just got better and better but at each stage of his career has delivered a great performance in at least one or two films when it's been needed to remind you what the fuss is about.