It's actually quite difficult to point to a specific answer because, as your Boudica example shows, in many cases casualty estimates are not very reliable. The 80,000 figure given for Boudica's forces is considered to be a rhetorical device and not an actual factual number - a problem that plagues many pre-modern accounts.
Another difficulty is measuring what counts for casualty. Historically, a great part of war losses comes from disease and illness, rather than direct combat. Moreover, there is no unified standard for counting what passes for "wounded" in historical records.
Nonetheless, here are some contenders:
Operation Desert Storm
Among larger set piece battles, this is probably the clear winner. Coalition forces suffered about 1,155 casualties (379 deaths and 776 wounded). Iraqi casualties are less clear, but early claims of 100,000 fatalities are wildly implausible. A more reasonable estimate would be 26,000 killed or wounded with about 83,000 captured. Even with this conservative estimate, the ratio remains an astonishing 1:94. If prisoners are excluded, this ratio falls to a still high 1:22.5.
Desert Storm also contained a number of heavily lopsided engagements, most notably the Highway of Death during which Coalition forces suffered no casualties, giving an effectively infinite ratio.
Battle of Omdurman
The Anglo-Egyptian-Sudanese force took about 430 casualties (48 dead and 382 wounded). The army of Abdullah al-Taashi lost around 30,000, including 12,000 dead, 13,000 wounded and 5,000 captured. This gives a ratio of 1:69, or 1:250 if we only count fatalities.
Battle of Majuba Hill
Probably the most lopsided result in a relatively evenly matched engagement. The Boers lost only 1 dead and 5 wounded, but managed to inflict 92 fatalities, 134 wounded and 59 captured on British forces. This is a ratio of 1:47.5, or 1:92 if we only count fatalities.
It is very rare for pre-industrial battles to achieve ratios like Desert Storm or colonial skirmishes at any scale, although some total disintegrations of the defeated side did occur. In such cases casualties are not necessarily combat deaths or injuries or captured, but often also from exposure during a long rout or simply desertion. Some contenders are:
Battle of Tumu
A 230,000 or 500,000 (estimated actual size vs paper strength) strong Chinese army led by the Ming Emperor was destroyed by a Oirat cavalry force of around 20,000. Precise casualties are not known, but Chinese records indicate that the expedition was entirely annihilated. Even assuming they costed the Oirats extremely dearly, the ratio couldn't have been any lower than 1:23 or so, and might be (but probably isn't) as high as 1:166 going by Wikipedia figures.
Battle of Fei River
A Jin force about 80,000 was able to crush an 870,000 invading Qin force through inducing a panicked rout. Again detailed casualty numbers are unavailable, however the Qin Emperor was recorded to have returned with only 20-30% of his original force. Traditional sources indicate minimal Jin casualties, though some online sources claim 5,000 with no clear evidence. Conservatively estimating a Qin 600,000 casualty, most of which incurred during the rout, and assuming a more reasonable 10,000 lost for the Jin, that still gives a ratio of 1:60.
I feel this deserve to be mentioned. During the Mongol Empire's expansion into Europe and elsewhere, they fought several militarily wildly successful battles that annihilated entire armies. For example, the Battle of the kalka River, or the Siege of Bagdad, and the Battle of Liegnitz. The ratios are particularly lopsided because the Mongolians could chase down routed enemies with great lethality. However, there are usually no reliable estimates for Mongolian casualties, so no real ratio can be calculated.
Other honourable mentions:
Battle of Kircholm
One of the most successful heavy cavalry charges of all time. The Polish-Lithuanians are said to have suffered about 300 casualties (100 dead and 200 wounded); the Swedish army however lost between 7,600 to 8,900 men, most killed or executed as they were routed. That's a ratio of 1:25-30.
Battle of Rorke's Drift
Example of lopsided victories for a heavily outnumbered force. About 150 British and colonial soldiers held off a 3,500 strong Zulu force, inflicting 351 confirmed kills for 17 fatalities in the process. Including 15 British and an estimated 500 Zulu wounded, the ratio is 1:26.6.