My question is: Do you truly mean antivirus software, or antimalware software? There is a difference; AV is virus only, though many AV's now detect rootkits/trojans, so they are really hybrids. Antimalware means any malicious execution under the sun: worms, rootkits, viruses, trojans, ransomware, keyloggers, droppers, etc.
Finding a true, or hybrid (detects things other than viruses) AV on-demand scanner with a cloud is going to be tough, especially if you are looking for freeware.
Biozfear says HitMan Pro, which is excellent for on-demand scanning for all malware, and the cloud works beautifully in my humble opinion. Once you activate the free trial, you have 30 days of free removals to your credit. After that, you can use "early warning detection" and click "goto location" on the drop down menu of any give line item to manually delete the malware. Beware though, early warning detection makes false positives much more common, and I don't know if the cloud will work unless you are paying or else you are in the midst of the 30 day trial. There is also forced breach mode which kills(forces shut) processes that interfere with removal and detection.
I don't know about it having a cloud, but McAfee Stinger is what I would consider a traditional AV on-demand scanner.
Dr. Web Cureit is the one I trust (its engine is included as one of 5 in HitMan Pro). I run it once a month to check for worms and viruses, though it too is a hybrid "AV" scanner; it checks for all malware.
Malwarebytes Anti-rootkit (MBAR) is going to be huge. It is already looking pretty good, and has a repair tool to fix catastrophic problems that can be associated with malware removal.
Avast has a free anti-rootkit utility that is supposed to be capable of repairing your master boot record when bootkits change/damage it.
The standard, for novice and expert alike, seems to be MBAM and HitMan Pro. Most of us have those for front line on-demand scans. I run MBAM once a week and HitMan Pro once a day. HitMan rarely takes more than 3 minutes to scan. MBAM takes about 15 or 20 minutes for a "full scan".
Bitdefender has a browser add-on for Mozilla's Firefox that is an online scanner---Bitdefender QuickScan 0.9.9.119. It is fast. Bitdefender is also an engine that HitMan Pro utilizes.