I've been around here on BoardGameGeek for I don't know how long. A few years. I just got some microbadge for it. But I've never given a game a 10 rating.
If I go by the recommended criteria for the ratings, I just couldn't do it. 10 says you like the game and think you always will. A lot of people will give out 10s promiscuously. They might give a 10 the first day or week they try it. I don't think after a week of play you can know you won't get tired of a game. By the rules of 10, you should never lower a 10 rating, or it was never really a 10 in the first place. I bet not a lot of people can say they haven't lowered any of their 10 ratings, or if they're honest they should. If a game is sitting in your closet a year without being played, it isn't a 10 any more.
The other thing is I felt like 9 was adequate for distinguishing my favorite games. I liked them all about the same. None had to be rated higher than the others.
Well, I'm giving Pandemic: On the Brink a 10 now. After nearly 2000 games on BSW, I still find every play fresh. The difficulty goes high enough to keep me from winning too easily, but it doesn't just beat you regardless of all your efforts. Some games are unwinnable, but I still think every little thing you do a bit more cleverly can knock your chance of winning up a percentage point or two.
My first 10 and it's an expansion? Typically I give expansions lower ratings than the base game. Which is not to say I like the game less with the expansion, just that the expansion doesn't add as much to my gaming fun as the introduction of the base game did. It improves slightly something awesome. For Pandemic, however, On the Brink is essential to its 10-ness. I'm not sure I'd still be playing it on Heroic difficulty every time. I got good enough to win that when it was winnable I think. It wasn't uncommon for me to clear the board.
The Legendary difficulty is pretty much just right, and if I want a bigger challenge, I add mutation. Or I mix it around. Heroic with mutation sometimes.
I've found Legendary with mutation to be pretty winnable if you keep to the original roles. The expansion roles are not bad, just more closely spaced. Almost any pair of the original roles will be somewhat balanced. Many pairs of roles when you include the expansion will overlap a lot, and you have a lot of trouble dealing with the issues that aren't their forte. For example, a Medic and a Containment Specialist will do great at treating cubes, but quite badly at finding cures and moving around the board. The Archivist and Troubleshooter have a lot of overlap. The Troubleshooter can fly somewhere without discarding the card; the Archivist can do the same for 1 more action. There are other examples. It's less of a problem with more than two players, because it's hard to get stuck with a set of three roles that all do substantially the same thing.
I'm not really complaining about the expansion roles making the game a little bit harder. That's a great option to have. I can always scale back the difficulty and not play with Legendary and mutation and expansion roles, though most of the time I do play with all three. More than doubling the number of roles has exploded the possible combinations. Whereas in the base game I had kept track and won on Heroic with every possible combination of roles (at least pairs), I wouldn't even start to try to track it with all the expansion role combinations. There are dozens with two players and thousands with four players. Every combination makes for a different experience.
Besides all the official ways to vary the game, the game lends itself to unofficial rules modifications that still leave the game playable, though with some difference in difficulty. One I've tried a bit is a "civilian" game, where you don't use any special abilities. It's a lot harder! I also like to try variants on how much information sharing is allowed. People debate a lot about what is the "right" level of information sharing to use, but I say, try any you can think of for a different challenge.
All the variations are what make the game great, but that can be said about a lot of games. What makes Pandemic replayable squared is that the way random cities and epidemics appear on the board, and your random card draw for cures and flights, present a different never-before-seen challenge in every game.