Free VPNs often tend to be slow (which means that you can't reliably stream video, for example) or they come with data limits (so that if, say, you want to access videos via them, you can only watch one or two a month). Some also force you to accept intrusive advertising as well. So none of them are perfect. The best though are summarised here:
https://www.techradar.com/uk/vpn/best-free-vpn
The REASON you want to use a VPN is also relevant. If you simply want to keep your IP address a secret then it won't matter where the VPN has servers. (i.e. it will make no difference at all as to whether the VPN makes your connection look as if it's coming from the USA or Uganda). However if you're trying to access the output of a US TV service, you'll need to use a VPN that's got servers in the USA. (Similarly, if you're trying to watch TV from Singapore, you'll need to use a VPN that's got servers in Singapore).
If, say, you want to access an American video service, it would also be best to use a VPN that's got MULTIPLE servers in the USA. The video service provider might block access from one or more VPN addresses in their country, so it's important to have other servers in that country to try too.
If there's a reason why you might not want, say, the police or security services to be able to trace your IP address through the VPN, then you should seek out a VPN that doesn't keep logs of traffic passing through their servers. (However it would be unwise to trust many VPNs that claim not to keep traffic logs. A gang of Mexican drug smugglers were using a Canadian VPN to pass all their internet traffic through. That VPN claimed not to log any traffic but, when served with a court order, they still handed over the IP addresses being used by the gang). Anyone who seriously needs to keep their IP address away from 'the authorities' should forget about using a VPN and, instead, use the Tor network. (However Tor isn't fast enough to handle streaming traffic, such as video services).
The ONLY way that you'll get a really reliable VPN service, with fast speeds, no data limits and no data logs, is to PAY for it. NordVPN frequently comes out top in comparison tests carried out by the technical press and online review sites. It works out at under £3 per month when you sign up to their initial two-year deal:
https://nordvpn.com/