You'll get the most mileage out of anything centered around a personal goal or interest -- the more personal and genuine and less superficial the goal or interest is, the better. As an example, schools (as a student or postgrad) and religious groups (if you're religious) are very good if you're genuine in your involvement.

Also, social events connected in some way to people you already know (both family and friends) provide an incredibly high degree of optionality in all sorts of ways, including potential relationships.

As others have noted, besides improving yourself and becoming a person people would want to be with, a very effective method of finding a lifelong romantic relationship is to run the relational equivalent of a sales funnel. (1) Be intentional/strategic about meeting a lot of new people using methods mentioned above; even keep a monthly tally and try to increase it past 5, 10, or even 20 per month (where "meet" means you have a conversation where you exchange names and talk a little). (2) Be willing to go on a date with anyone who's safe and interested once. You'll likely be surprised and discover that some types of people are a lot more interesting than you had thought previously. (3) Continue dating a person you're interested in after the first date, until you decide that person isn't going to be it. The more people there are at the top of the funnel, the more selective you can be, and the less you'll be tempted to commit to someone too early just because you want to be in a relationship.