I realise this is very subjective and is open to criticism but I thought it could be an interesting topic. I decided to try and compare all the cars over the years and then make a figurative lap time and see what would have been the quickest over time. I've compared what I considered to be the best cars of each season (that's open for discussion but the fact is what you may think was a quicker car may only have been a few tenths quicker so for the point of the exercise it doesn't really matter), I've compared each car against the car best car from the previous season on the same track, therefore getting the difference in percentage time of each other. Then I've kept a running percentage total which this is multiplied against, therefore eventually comparing the 1950 Alfa Romeo 158 against the 2011 Red Bull RB7 Renault.

So in year order:

CAR YEAR FIG LAP TIME
Alfa Romeo 158 -1950 2:00.000
Alfa Romeo 159 -1951 1:55.410
Ferrari 500 -1952 2:03.998
Maserati A6SSG -1953 1:58.519
Mercedes W196 -1954 1:56.603
Mercedes W196 -1955 1:55.046
Lancia Ferrari D50-1956 1:53.824
Maserati 250F 1957 1:53.935
Vanwall 1958 1:52.259
Cooper T51 Climax 1959 1:51.109
Lotus 18 Climax 1960 1:47.888
Ferrari 156 1961 1:50.074
BRM P57 1962 1:46.508
Lotus 25 Climax 1963 1:45.358
Lotus 25 Climax 1964 1:45.013
Lotus 33 Climax 1965 1:44.668
Brabham BT19 Repco 1966 1:41.332
Lotus 49 Ford 1967 1:37.307
Lotus 49 Ford 1968 1:36.432
Lotus 49B Ford 1969 1:32.993
Lotus 72 Ford 1970 1:30.290
Tyrrell 003 Ford 1971 1:29.301
Lotus 72D Ford 1972 1:27.450
Tyrrell 006 Ford 1973 1:28.294
Ferrari 312B3 1974 1:26.849
Ferrari 312T 1975 1:26.395
Ferrari 312T2 1976 1:28.211
Lotus 78 Ford 1977 1:26.956
Lotus 79 Ford 1978 1:24.404
Ferrari 312T4 1979 1:22.669
Williams FW07B Ford 1980 1:19.688
Williams FW07C Ford 1981 1:22.871
Renault RE30B 1982 1:16.245
Brabham BT52 BMW 1983 1:18.112
Brabham BT53 BMW 1984 1:15.185
Lotus 97T Renault 1985 1:14.174
Williams FW11 Honda 1986 1:12.681
Williams FW11B Honda 1987 1:13.001
McLaren MP4-4 Honda 1988 1:14.022
McLaren MP4-5 Honda 1989 1:13.055
McLaren MP4-5B Honda 1990 1:10.685
McLaren MP4-6 Honda 1991 1:09.545
Williams FW14B Renault 1992 1:09.515
Williams FW15C Renault 1993 1:09.709
Williams FW16 Renault 1994 1:09.265
Williams FW17 Renault 1995 1:10.121
Williams FW18 Renault 1996 1:09.062
Williams FW19 Renault 1997 1:07.873
McLaren MP4-13 Mercedes 1998 1:08.037
McLaren MP4-14 Mercedes 1999 1:10.174
Ferrari F1-2000 2000 1:08.675
Ferrari F2001 2001 1:06.284
Ferrari F2002 2002 1:05.384
Ferrari F2003-GA 2003 1:06.350
Ferrari F2004 2004 1:04.432
McLaren MP4-20 Mercedes 2005 1:05.155
Ferrari F2006 2006 1:05.223
McLaren MP4-22 Mercedes 2007 1:04.948
McLaren MP4-23 Mercedes 2008 1:05.381
Brawn GP Mercedes 2009 1:06.093
Red Bull RB6 Renault 2010 1:05.384
Red Bull RB7 Renault 2011 1:04.440

What this list means is that if the 1950 Alfa Romeo 158 could do a lap time around a circuit of 2min, then the current 2011 Red Bull would do it in 1:04.440.

It's interesting from this chart to note the changes in regulations over the years and how that effected the figurative lap time and how development improved rapidly in the 70s and 80s.

So in theory the fastest F1 cars ever were the 2004 versions, followed by the current cars.