each scanner has its own strengths and weaknesses, there is no best. If you want the best, you must combine 2 scanners at least:
My best combo:
Internet is available: Zemana + norton power eraser
Internet is not available: Emsisoft emergency kit
Unbootable machine, rescue CD: ESET SysRescue
Reasons:
- Zemana: fully cloud, no need for update, always has the latest signatures, multi-engines, very fast, good signatures, the absolute best against worms, file-less malwares, and already infected PC, can detect and revert malware's changes effectively | Cons: doesn't scan all user data folder -> can miss some malwares but you can scan it manually
- Norton power eraser: checks the reputation of files so it doesn't need signatures -> can detect new malwares due to their poor reputation | Cons: only for advanced users because of false positive rate, many safe programs have low reputations
- HitmanPro: 4 engines (kaspersky, BD, sophos & their own engine), good detection rate, sometimes better than zemana, sometimes worse | Cons: extremely poor against file-less malwares, not the fastest scanner, requires license to use in long term in 1 machine
- Emsisoft emergency kit: dual engines, great signatures against non zero-day malwares, very fast speed | Cons: not the best against worms, file-less malwares, very new malwares, requires signatures update and the update takes very long time
-ESET SysRescue: fast scanning speed and good signatures | Other rescue CDs have very slow scanning speed (Kaspersky) or poor/basic/not the same signatures (bitdefender, avira) compared to their AV products