I think Fire Red is great. There was some work required to get a good team assembled. For example to get a good Scyther required getting two Scythers (pretty rare), mating them (because Silver Wind is an egg move) and hatching the egg, then training it up. It's a lot more involved when you have more involved base stats, such as Special Attack vs regular Attack, Speed, etc. A lot more strategy in my opinion.
Red/Blue were good for their time but it's too frustrating to play these days (the user interface is difficult and time consuming to use). Yellow was an improvement, but Fire Red is better, in my opinion, mostly because for R/B/Y when you win the game there is nothing to do. Fire Red lets you actually continue to play with the team you worked so hard to build (Trainer Tower, rematch trainers, searching for special items [E.g. trying to find a Twisted Spoon or Lucky Egg], etc.). Building a good team is fun, this is one of the better sites for doing so http://pokemondb.net/pokedex/stats/combo.
I'm a fan of first generation Pokemon, never really cared much for what came after that, so I don't have experience with any of the games beyond FR. One thing I don't like about FR is that Mew was only given out at very limited real-life events.
