Creatine
Creatine is the rare supplement with a genuinely strong evidence base for strength and power training, which is why it is so widely used. The good news for your wallet: plain creatine monohydrate is the well-studied one, and the fancy "advanced" versions rarely justify the premium. Powder is cheapest per serving; capsules trade a little cost for convenience.
Common questions
Is creatine safe?
It is one of the most studied supplements around and is considered safe for healthy adults at the usual doses. The old kidney worries have not held up in the research for people without existing kidney problems.
Do I need to "load" creatine?
You can, but you do not have to. Loading fills your stores faster; taking a steady 3 to 5g a day gets you to the same place in a few weeks with less faff. Monohydrate is all you need.