Below are listed, in order of apparent brightness (magnitude), the brightest comets observed by ground-based observers since 1935, chiefly as documented in the ICQ archive and IAU Circulars (but also consulting the reports in the Harvard College Observatory Announcement Cards, QJRAS, Sky and Telescope, Observatory, PASP, and JBAA). The "peak m1" is the maximum observed total visual magnitude as seen by ground-based observers; this is an average value determined from the most experienced observers [in other words, some brighter magnitudes have been excluded for some comets; the paucity of observers working to make good magnitude estimates many years ago, and the problems involved when comets are very near the sun and/or are much brighter than available comparison stars/planets, make it difficult to determine what the peak values should have been (this is also why we chose the cut-off to be 1935, instead of an earlier date)]. A good example of this is regarding the bright "Eclipse comet" of 1948, which was stated by two observers to have a nuclear condensation brighter than Venus, but to be several magnitudes fainter at brightest by most other accounts; so the uncertainty in peak total visual magnitude is something like -2 ± 3 magnitudes! [See also Bortle's "Bright-Comet Chronicles".] Peak magnitudes in parentheses are more highly uncertain (or not completely determined). Only those comets brighter than fourth magnitude are listed below. Note that some comets have been seen brighter than listed below in spacecraft images when the comets were too close to the sun for ground-based observation; for example, C/2012 S1 was seen to peak around mag -1.9 in SOHO solar-coronagraph images and comet 96P/Machholz was seen to peak around mag 2.0 in SOHO images from Octo. 2017.
One can see from this that, on average, we are able to view a comet that is brighter than fourth magnitude once every two years, a comet brighter than second mag once every 5.5 years or so, a comet brighter than first magnitude every 10 years or so, and a comet brighter than mag 0 roughly once every 15 years; of course, sometimes there will be two bright comets in a span of a year, while at other times there will be a 20-year interval between such apparitions. [updated 2020 July 5]
peak m1 Comet ---- ---------------------- (-10) C/1965 S1 (Ikeya-Seki) (-5.5) C/2006 P1 (McNaught) -3.0 C/1975 V1 (West) (-3) C/1947 X1 (Southern comet) (-1) C/1948 V1 (Eclipse comet) (-1) C/2011 W3 (Lovejoy) -0.8 C/1995 O1 (Hale-Bopp) (-0.5) C/1956 R1 (Arend-Roland) (-0.5) C/2002 V1 (NEAT) 0.0 C/1996 B2 (Hyakutake) 0.0 C/1969 Y1 (Bennett) (0) C/1973 E1 (Kohoutek) (0) C/1962 C1 (Seki-Lines) 0.5 C/1998 J1 (SOHO) 1.0 C/1957 P1 (Mrkos) (1.0) C/2011 L4 (PANSTARRS) (1) C/1970 K1 (White-Ortiz-Bolelli) (1.5) C/2020 F3 (NEOWISE) 1.7 C/1983 H1 (IRAS-Araki-Alcock) (2) C/1941 B2 (de Kock-Paraskevopoulos) (2.2) C/2002 T7 (LINEAR) 2.4 1P/1982 U1 (Halley) (2.4) 17P (Holmes) [Oct. 2007] 2.5 C/2000 WM_1 (LINEAR) 2.7 C/1964 N1 (Ikeya) 2.8 C/2001 Q4 (NEAT) 2.8 C/1989 W1 (Aarseth-Brewington) 2.8 C/1963 A1 (Ikeya) 2.9 153P/2002 C1 (Ikeya-Zhang) 3.0 C/2001 A2 (LINEAR) 3.3 C/1936 K1 (Peltier) (3.3) C/2004 F4 (Bradfield) 3.5 C/2004 Q2 (Machholz) 3.5 C/1942 X1 (Whipple-Fedtke-Tevzadze) 3.5 C/1940 R2 (Cunningham) 3.5 C/1939 H1 (Jurlof-Achmarof-Hassel) 3.5 C/1959 Y1 (Burnham) 3.5 C/1969 T1 (Tago-Sato-Kosaka) 3.5 C/1980 Y1 (Bradfield) (3.5) C/1961 O1 (Wilson-Hubbard) (3.5) C/1955 L1 (Mrkos) 3.6 C/2012 S1 (ISON) 3.6 C/1990 K1 (Levy) 3.7 C/2014 Q2 (Lovejoy) 3.7 C/1975 N1 (Kobayashi-Berger-Milon) 3.9 46P/Wirtanen [Dec. 2018] 3.9 C/1974 C1 (Bradfield) 3.9 C/1937 N1 (Finsler)
More from the ICQ archive: During 1950-2003, there were three documented cases of two comets being observed via naked eye at the same general time (though not necessarily from the same geographical location or same instantaneous time) -- all three cases being with both comets at total visual mag 5-6 at the time. There were more than a dozen additional cases during that 5-decade span in which two (or sometimes even three) comets were being observed at the same general time where both (or all three) comets were at mag 6.5 or brighter. Some of these may indeed have been detected via naked eye, though magnitude estimates are not available in the archive to substantiate this. The example of C/2001 Q4 and C/2002 T7 being both brighter than fourth magnitude during a period of time in 2004 April-May, joined unexpectedly and briefly by C/2004 F4, is thus highly unusual.