Well, if we’re talking about tournaments anyone can participate in, you can get no bigger than the annual league conferences, anonymous. You may already know this, but each year, participants in every region’s Pokémon League who have earned a minimum of eight badges gather together to battle and determine who’s the best of the best for that particular year. While each of these tournaments are equally challenging, I must say I’m a little partial to Johto’s Silver Conference and Kanto’s Indigo Plateau Conference myself. (Then again, I am also horrendously biased, so take this opinion with a grain of salt. My partner, who is slightly less biased, considers both to be dull and finds the Lumiose Conference far more interesting for reasons I believe have to do with the fact that Professor Sycamore is often in attendance.)
[That would be your reason for watching it with me. —LH]
Outside of the conferences, there are also annual exhibition matches in every region between members of the Elite Four, plus the world-famous Pokémon World Tournament of Unova, which pits gym leaders against one another and the champion of Unova. Additionally, under very special circumstances, a trainer who’s earned eight badges may go up against a region’s Elite Four and champion in the Elite Four Gauntlet, which is frequently a televised event as well (mostly because the winner of the gauntlet is traditionally crowned the next regional champion).
Additionally, if you’re willing to stretch the definition of “battling,” coordinators also get their own version of conferences with the grand festivals, such as the Hoenn Grand Festival, which is the oldest (although it’s commonly thought that the Wallace Cup is both bigger and more exciting than any grand festival in existence).
As for requirements, practically anyone can participate in the league conferences and grand festivals. All you need is the appropriate number of badges or ribbons from officially sanctioned gyms and contest halls to enter. Traditionally, this means eight badges or five ribbons, but there are exceptions. (Competing in the Orange Islands’ Champion Battle, for example, only requires that a trainer earn four badges before arriving at Pummelo Island. The Wallace Cup, meanwhile, has no such requirement at all, as its ribbon instead counts towards any circuit’s requirements for entry into the regional grand festival.) Meanwhile, the exhibition matches are obviously not open for the public and are instead performed between select groups of trainers, usually Elite Four or gym leaders. There was only one recorded instance of a regular trainer participating in the Pokémon World Tournament, but this was more of an extenuating circumstance.
The gauntlet, meanwhile, is a bit trickier to get into. While in theory, anyone can join, in practice, it involves not only earning the eight badges but also clearing the Victory Road challenge and then … arranging a battle with all five members of the gauntlet. Yes, I realize that sounds anticlimactic, but when it comes to the Pokémon League, there is no bigger challenge to a trainer than utter bureaucracy.
(On a serious note, Victory Road is traditionally the most dangerous place in each region, so by the time you’ve gotten through to the end, you’ve more than proven yourself worthy of a battle with the gauntlet. This is why the next step is pretty much getting them to agree to battle you.)
Besides those, there are of course local tournaments, such as Johto’s Whirl Cup (a tournament for water-type specialists that takes place around the Whirl Islands), but these tend to have fanbases that are confined to their respective regions, even if some of them (such as the Whirl Cup) are almost as large as the regional conference. Practically anyone can participate in these tournaments, but oftentimes, they’ll have certain rules that restrict trainers to specific types or specific kinds of pokémon. For example, the Whirl Cup, as a tournament exclusively for water-type specialists, only allows you to battle with water-types, although you only need a minimum of two to participate. (The first round involves a one-on-one match, whereas the second is two-on-two. Of course, it’s best to have a full team of six to choose from when you register for the tournament, but you could quite literally get away with the minimum number of two and still win.) Other tournaments, of course, vary rather wildly, so chances are if you’ve heard of it on your travels through each region, it may be worth it to look into—and even then, be sure to check the rules thoroughly before entering.
Best of luck!
The Pokédex According to Bill 