Generation 5 is one of my favorites, though not my absolute favorite.
The first and major thing it has in its favor is that it was the first generation where I was able to trade Pok?mon online. I had a lot of fun with that, including trading myself the other starters and managing to do trade evolutions for the first time since Pokemon Stadium 2. Speaking of online, when it was operational it was pretty fun to use the Pok?mon Dream World on the Pok?mon website and find Hidden Ability Pok?mon, though that could get annoying if you were trying to find something specific.
I personally enjoy the Pok?mon introduced in this generation, including Trubbish and Garbodor since they seem to get a lot of hate. There were a lot of cool ones like Archeops, Braviary, Chandelure, Eelektross, Excadrill, Galvantula, Gothitelle, Haxorus, Mandibuzz, Scolipede, Hydreigon, Sawsbuck, Volcarona, and Zoroark and it has some of my favorites of all generations like Beheeyem, Cofagrigus, Escavalier and Serperior.
I didn't mind the lack of Pok?mon from other regions because I had recently come back to the series with HeartGold & SoulSilver and I was more interested in trying out new Pok?mon I hadn't used before. Additionally, if you had more than one copy and the ability to transfer Pok?mon up into the Gen 5 from the Generation 4 games, or if you knew people who had the Pok?mon you wanted to use and could trade with them, it wasn't that big of a deal.
I enjoyed traveling across the Unova region and it had a lot of enjoyable stuff to do, and I really liked the addition of the Pok?mon World Tournament in Black 2/White 2. It's overall a very strong generation for me, and the only reason it's not my absolute favorite is because of the advances they've made in the year since. Things like Generation 6 introducing the ability to use the Destiny Knot to pass on IVs while breeding, Super Training in the Generation 6 games and the boost to the Power Items in Sun/Moon, the ever expanding Pok?Dex, and of course Trainer Customization. Needless to say, I'd absolutely love to see a remake of Generation 5 down the line with modern graphics and access to all of the Pok?mon up to now.