I have a laptop with Windows installed and iPhone both synchronised to Dropbox. Also I use Google Drive app on my phone and on my laptop I access Drive using browser. Which way of using cloud is more safe? Also I don't have as strong security on my laptop so can easily pick up some malware. The second question is: can it spread to my other devices if one device is infected? How can I protect myself in this case?
As for Dropbox vs. Google Drive, I don't know which one is more secure. I think it's a matter of personal preference.
If you have malware, files on your cloud drive can get infected by viruses, or encrypted by ransomware. The infected files are then synced to your phone, but it's highly unrealistic that they could do any damage there, since Windows and iOS are very different systems.
It is possible, though, for example by a PDF file that could trigger a buffer overflow in iOS. But then an attacker should know or guess that you use iOS.