There is no magic against "any kind of opponent," because your opponents are so potentially varied. Some are armed; others not. Some are individuals; others are groups or gangs. Some are simply angry; others are (pardon the now-obsolete psychological phrasing) psychopaths. Some are your drunk uncle Eddie, who you don't want to hurt, but you can't have him putting his hands on everyone at the family BBQ. Different weight classes, different weapons, different intentions--the variations are endless.
Pretty much any martial art will help train you in cardio fitness, in balance, in using your weight and strength more efficiently. But some--Western fencing, Kyūdō (Japanese archery), Iaido (the drawing and use of Japanese swords), or Kendo (Japanese swordsmanship with bamboo swords and armor) just to name names--however lovely and historical as arts, are simply not practical fight training for modern people. Similarly, in most Western countries, any art that assumes you will be always armed (e.g. Eskrima, Arnis and Kali) are just not practical. In most places you cannot carry bladed weapons. Even if you could/did, and then used them, you'd probably go to jail for slicing your opponent up in the ways they train/suggest.
For self-defense, I personally prefer training in arts that emphasize the the punching, trapping, and controlling range (arms-length or closer to your opponent) rather than kicking or grappling range. So, Japanese Jujutsu or its American Small-Circle derivative, say, rather than Taekwondo, wrestling, or BJJ. Some karate schools incorporate controls and throws in with striking moves, which is a nice mix. Some kickboxing or sport combat instructors who are savvy to MMA and BJJ teach similarly fluid styles that combine striking, control, and joint-locks. Any of these are preferable to the highly-refined, stylized arts.
There are also some martial arts that are specifically "combat engineered": simplified and refined, removing traditional flourishes and sport-fighting rules to focus only on defensive fighting. They're more "martial skills" than "martial arts." Systema and Krav Maga are often cited, though American Small-Circle Jujitsu, Combat Hapkido, and other variants are out there.
You've had wrestling experience, so why not focus on boxing, kickboxing, or jujutsu to extend your range/set of techniques? I know when I added kickboxing and Small Circle Jujitsu techniques to my bag of tricks (previously kung fu, Judo, and Eskrima), I felt enormously more capable in defensive situations.
Sardathrion makes an excellent suggestion about techniques like "No Defense Self Defense." They extend your awareness, posture, voice, and presentation, and are really the most effective defense you have. If you're looking for true self-defense training, it's not a matter of just "choosing a martial art." The "real world self defense" systems that incorporate awareness, the use of your voice (e.g. Verbal Judo), under-high-stress adrenaline dumps, and truly defense-focused techniques are worth seeking out. Rory Miller's Facing Violence: Preparing for the Unexpected is a good place to start to bridge the gap between "martial arts," sport fighting, and real-world violence dynamics and responses.
Here is a recent podcast on this topic: PREVAIL Podcast #3: Self Defense and the Martial Arts. (Full disclosure: I am one of the participants.)