1/ Zemana: free, portable (but with some traces), fast speed, easy removal process. Not good great against brand new executable malwares (.exe) but extremely useful against already infected PCs, fileless malwares and PUPs. In default scan option, it doesn't scan Users folder so this should be scanned after the default scan. Very good detection rate which tends to be more updated than EEK
2/ HMP: free to use but only free to cleanup for 14 days. Slower scanning speed and slow upload speed. Very good detection rate and tends to be the most updated scanner out there. Very poor against fileless malwares but quite effective against new .exe malwares (which zemana is not good at)
3/ EEK: free, very fast scanning speed, very effective with the best detection rate but requires signature update, which can take some time due to large size. Very good against .exe malwares but doesn't seem to be the most updated scanner. It seems to be quite poor if the malware is brand new (<1-2 days). HMP and Zemana are more updated, IMO. Effective if the PC is infected after 1-2 days
4/ Malwarebytes: good against trojans, ransomwares and PUPs (maybe the best or second best against PUPs) but there is no official portable version. Scanning speed is OK. Poorest detection rate against regular malwares. Not recommended
5/ Norton Power Eraser: free, very fast scanning speed. It's like a file reputation checker rather than a signature-based scanner. Files with a low number of users and files known to have poor reputation will be listed regardless of their safety so it's extremely prone to false positives and can be dangerous if the user cannot distinguish which file is safe, which is not safe. Due to their low number of users, malwares tend to be detected by NPE although they are all missed by the other signature-based scanners. Very helpful but use with caution
6/ ESET online scanner: relatively slower scanning speed than all the above. Very good detection rate and tends to be the most updated, much better than EEK (in the first 1-2 days when EEK hasn't got the signatures for the malwares yet). Super effective against PUPs in default mode and a bit aggressive if PUP detection is selected
I'm using the ones in bold