As Toronto's southernmost east/west thoroughfare prior to major infill efforts in the early part of the 20th century, it's not surprising that Front Street has been home to some of the city's most important buildings, at least a few of which remain to this day. Front has, at point or another, housed the province's early parliament buildings, Toronto's first city hall, the St. Lawrence Market, the Flatiron Building, multiple Union Stations and some of the city's finest hotels, to provide only a brief sample of its most important tenants.
Going back a couple of centuries, what we now call Front Street was referred to as Palace Street thanks to presence of the province's second parliamentary buildings. These were located at Front and, you guessed it, Parliament streets (etymology is fun!) before being rebuilt further west (near Simcoe) in 1832.
In addition to this important bit of provincial history, the street played a major role in the development of the city itself. Located on the current site of the St. Lawrence Market, Toronto's first city hall could be found at the corner of Front and Jarvis streets between 1845 and 1899, when it relocated to Queen and Bay Streets (what we now refer to as Old City Hall). The remains of this building have been preserved at the entrance to current the market, though the bulk of the structure was demolished long ago.
There are too many other iconic structures that have called Front Street their home to devote much time to each of them, but it'd be silly not to draw attention to the gem that was Old Union Station. Technically the city's second hub train station (there was apparently a first, far less dramatic Union prior to this one's construction), it sat on Front between York and Simcoe streets until it was demolished in 1931. Perhaps it's a stretch to say that the loss of this building was equivalent to that of the original Penn Station in New York, but not by much.
Speaking of Front Street structures lost to demolition, it still irks me that the Board of Trade Building was knocked down in 1958 only to exist as a parking lot prior to the construction of the EDS Building, which wrapped up in 1982. Toronto! What were you thinking?
Also, noteworthy, of course are the Royal York Hotel, The Cyclorama Building, and current day Union Station, but we've already written about them, so I'll stop blathering in favour of presenting the photos.
MAP

Goad's Atlas, 1910
PHOTOS

Third Parliament Buildings (Front & Simcoe), 1834

Front Street, 1876

Coffin Block, 1888

Coffin Block, 1880s

Gooderham Building (Flatiron), 1890s

Victoria Row, 1890s

Bank of Montreal at Yonge and Front (now the Hockey Hall of Fame), 1900

Board of Trade Building (NE corner Yonge & Front), 1900

Aftermath of the 1904 fire, looking west across Front

Flagpole painter (view of Front looking west from Yonge), 1907

Old Union Station (a.k.a. Union Station II), 1910s

Queen's Hotel (Front & York), 1915

Cyclorama building, 1922

Union Station, 1926

Royal York Hotel (Front & York), 1929

Union Station, 1931

Streetcar in front of Union Station, 1932

Barclay Hotel (Front and Simcoe), 1955

O'Keefe Centre under construction, late 1950s

Opening night O'Keefe Centre, 1960

Yonge and Front, 1960s

St. Lawrence Market, 1965 (York University Archives)

St. Lawrence Market (canopy between north and south buildings), 1960s

Front east of Yonge, 1960s

Front east of Yonge 1970s

Walker House Hotel (Front and York), 1975

Cyclorama building, 1970s

Union Station, 1970s

Gooderham Building (Flatiron), 1970s

Dominion Public Building, 1970s

Bird's eye view of the St. Lawrence Market area, early 1980s

CBC Headquarters construction, 1988