Okay Don, here's my list. See if my ratings sync up with yours. I've constructed the list per IMDB's listing, and removed any television or voice-work, as well as very minor (uncredited) parts. I fully realize that voice-acting is somewhat demanding, but it's not really the same as real movie acting. Television is really about run of the mill. Early flashes of brilliance with "Mork & Mindy" and Jonathan Winters specials gave way to appearances on "Friends" and dramas.

See what you think:
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Robin Williams Filmography & Ratings

1. The Big White (2005) .... Paul Barnell N/A (not rated)
2. House of D (2004) .... Pappass 65/100 (mediocre)
3. The Final Cut (2004) .... Alan W. Hakman 72/100 (average)
4. Insomnia (2002) .... Walter Finch 70/100 (lackluster)
5. Death to Smoochy (2002) .... 'Rainbow' Randolph Smiley 44/100 (poor)
6. One Hour Photo (2002) .... Sy Parrish 78/100 (engaging, but weak)
7. Bicentennial Man (1999) .... Andrew Martin 8/100 (unwatchable; beyond torture)
8. Jakob the Liar (1999) .... Jakob Heym/Narrator 50/100 (mildly bad)
9. Patch Adams (1998) .... Hunter 'Patch' Adams 35/100 (painful)
10. What Dreams May Come (1998) .... Chris Nielsen 42/100 (not good)
11. Good Will Hunting (1997) .... Sean Maguire 66/100 (wishy-washy)
12. Flubber (1997) .... Professor Philip Brainard 30/100 (very painful)
13. Deconstructing Harry (1997) .... Mel/Harry's Character 45/100 (poor)
14. Fathers' Day (1997) .... Dale Putley 27/100 (abomination)
15. Hamlet (1996) .... Osric 36/100 (painful)
16. The Secret Agent (1996) (as George Spelvin) .... The Professor N/A (not rated)
17. Jack (1996) .... Jack Charles Powell 2/100 (approaching worst of all-time)
18. The Birdcage (1996) .... Armand Goldman 88/100 (very good)
19. Jumanji (1995) .... Alan Parrish 37/100 (hurting)
20. Nine Months (1995) .... Dr. Kosevich 40/100 (forgettable)
21. Mrs. Doubtfire (1993) .... Daniel Hillard/Mrs. Euphegenia Doubtfire 25/100 (abomination)
22. Being Human (1993) .... Hector N/A (not rated)
23. Toys (1992) .... Leslie Zevo 11/100 (torture)
24. Shakes the Clown (1992) (as Marty Fromage) .... Mime Class Instructor N/A (not rated)
25. Hook (1991) .... Peter Panning/Peter Pan 76 (passably good)
26. The Fisher King (1991) .... Parry 92/100 (greatish)
27. Dead Again (1991) .... Doctor Cozy Carlisle 39/100 (forgettable)
28. Awakenings (1990) .... Dr. Malcolm Sayer 57/100 (lamentable)
29. Cadillac Man (1990) .... Joey O'Brien 18/100 (Jay Leno-esque)
30. Dead Poets Society (1989) .... John Keating 84/100 (good)
31. The Adventures of Baron Munchausen (1988) (as Ray D. Tutto) .... King of the Moon 15/100 (oh the humanity)
32. Good Morning, Vietnam (1987) .... A2C Adrian Cronauer 96/100 (all-time favorite)
33. Seize the Day (1986) .... Tommy Wilhelm N/A (not rated)
34. Club Paradise (1986) .... Jack Moniker 21/100 (against the Geneva Convention - bad)
35. The Best of Times (1986) .... Jack Dundee 67/100 (average)
36. Moscow on the Hudson (1984) .... Vladimir Ivanoff 88/100 (very good)
37. The Survivors (1983) .... Donald Quinelle 36/100 (very hurting)
38. The World According to Garp (1982) .... T. S. Garp 90/100 (outstanding)
39. Popeye (1980) .... Popeye 90/100 (outstanding)
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and per my earlier comparison:

Steve Martin Filmography & Ratings

1. The Pink Panther (2006) .... Inspector Jacques Clouseau 38/100 (terrible)
2. Cheaper by the Dozen 2 (2005) .... Tom Baker 13/100 (torture)
3. Shopgirl (2005) .... Ray Porter N/A (not rated)
4. Meat Pie (2005) .... Oofta-Poofter #2 N/A (not rated)
5. Cheaper by the Dozen (2003) .... Tom Baker 26/100 (dreck)
6. Looney Tunes: Back in Action (2003) .... Mr. Chairman 40/100 (forgettable)
7. Bringing Down the House (2003) .... Peter Sanderson 19/100 (torture)
8. Novocaine (2001) .... Dr. Frank Sangster 37/100 (better to watch paint dry)
9. Joe Gould's Secret (2000) .... Charlie Duell N/A (not rated)
10. Bowfinger (1999) .... Bobby Bowfinger 32/100 (very painful)
11. The Out-of-Towners (1999) .... Henry Clark 26/100 (abomination)
12. The Spanish Prisoner (1997) .... Julian 'Jimmy' Dell 64/100 (mediocre)
13. Sgt. Bilko (1996) .... Master Sergeant Ernest G. Bilko 19/100 (torture)
14. Father of the Bride Part II (1995) .... George Banks 40/100 (forgettable)
15. Mixed Nuts (1994) .... Philip 60/100 (bad)
16. A Simple Twist of Fate (1994) .... Michael McCann 51/100 (mildly bad)
17. Leap of Faith (1992) .... Jonas Nightengale 71/100 (average)
18. HouseSitter (1992) .... Newton Davis 56/100 (sad)
19. Grand Canyon (1991) .... Davis 48/100 (pretty bad)
20. Father of the Bride (1991) .... George Stanley Banks 36/100 (terrible)
21. L.A. Story (1991) .... Harris K. Telemacher 82/100 (good)
22. My Blue Heaven (1990) .... Vincent 'Vinnie' Antonelli 71/100 (weak, but passable)
23. Parenthood (1989) .... Gil Buckman 80/100 (above average)
24. Dirty Rotten Scoundrels (1988) .... Freddy Benson 84/100 (good)
25. Planes, Trains & Automobiles (1987) .... Neal Page 87/100 (very good)
26. Roxanne (1987) .... C.D. Bales 93/100 (great)
27. Little Shop of Horrors (1986) .... Orin Scrivello, DDS 92/100 (greatish)
28. ¡Three Amigos! (1986) .... Lucky Day 46/100 (poor - but Martin Short factored into score)
29. Movers & Shakers (1985) .... Fabio Longio N/A (not rated)
30. All of Me (1984) .... Roger Cobb 79/100 (okay-good)
31. The Lonely Guy (1984) .... Larry Hubbard 87/100 (good)
32. The Man with Two Brains (1983) .... Dr. Michael Hfuhruhurr 96/100 (excellent-high honors)
33. Dead Men Don't Wear Plaid (1982) .... Rigby Reardon 90/100 (very good)
34. Pennies from Heaven (1981) .... Arthur Parker 85 (good)
35. The Jerk (1979) (also as Pig Eye Jackson) .... Navin R. Johnson/Cat, Juggler/Pig Eye Jackson/Engineer Fred 92/100 (very good)
36. The Muppet Movie (1979) .... Insolent Waiter 74/100 (average)
37. Sgt. Pepper's Lonely Hearts Club Band (1978) .... Dr. Maxwell Edison 87 (greatish)
38. The Absent-Minded Waiter (1977) .... Steven 90/100 (very good)