I once asked a NASA scientist if he thought there was life on Mars. He said "Yes, but we put it there." (He was referring to the fact that we did not realize when we sent the first probes that some microbes can withstand hard vacuum for quite some time, so we didn't bother to decontaminate them and may well have introduced hardy microbes to the Martian environment).
Realistically, though, where are we most likely to find life? Or rather, life as we know it.
Venus - Venus could have supported life during the first 2 billion years or so of its life. Modern Venus, though? It's a hellworld. 870F surface temperature, 92 bars of atmospheric pressure, no rain...it seems highly unlikely that any of that early life survived. However, there are spots in Venus' atmosphere that are almost homelike, and there is a not completely infeasible possible colonization plan involving airships (Alastair Reynolds includes this in On The Steel Breeze). It's possible there may be our kind of life drifting in the atmosphere - likely nothing complex, but...
Mars - We've been looking for life on Mars for a long time. It's an easy place to look. But if there is life, it's likely to be microbes clinging on underground or in clefts. Some researchers are looking at life in a salt mine in North Yorkshire, which might closely resemble the Martian underground. Is there anything there? Nothing that could build canals. But maybe microbes. (And maybe that scientist was right and we already contaminated the place).
Mercury - The general consensus is nope, there's nothing on Mercury. It's just too inhospitable, especially with daily temperature swings from -280 to 800 degrees Fahrenheit. (Which makes exploring it a challenge).
So, that's the three terrestrial planets other than Earth out of the way. Maybe we need to be looking a little bit further out?