By Jiriku of course
Jiriku's summoner is an exciting take on one of my favourite schools of magic. If one were to take everything about a core spellcaster and boil them down until only summoning remained this would be the pure, potent result. This class is crafted with care, effort, and artistry to deliver the perfect summoning experience.
Firstly the basic chassis is precisely what it needs to be. The basic framework is a wizard in every way save for having a D6 instead of a D4, which pathfinder players will find quite normal. Even that small change hints at what is so special about this class, thought has been put into it. A D6 instead of aD4 in 3.5 means that the Summoner will have more health than a normal caster, but why? The answer is one of the class' core abilities.
Planar companions. This class can take any summonable monster any essential appoint them as a familiar with all the benefits that entails. As the class gains levels they can have more than one such creature but only one can be summoned at a time. This does three crucial things at the same time. Firstly it guarantees the summoner is never alone. This companion can be present for a very long time and can be called to aid the summoner easily, that means the summoner can always play like a summoner, relying on bound creatures for their strength. Secondly this allows the summoner to make that particular creature much stronger. Allowing it to shore up any weaknesses by gaining familiar benefits. Since a familiar can use half its masters HP instead of it's own the class was given a D6 to make that ability more relevant, and it gives the class some extra survivability since they have limited defensive options. Lastly the planar companion can give some of it's own traits to the summoner which also helps grant survivability and versatility to a class with a spell list that, intentionally, grants only small amounts of either.
This class is filled with little bits of insight that show a real understanding of how the class will be used. From guaranteeing your summons will stay around for a few turns even at first level, to making sure you can communicate with more of your summons, to allowing you to call them faster there is a great deal of attention to detail. While the class is focused primarily on the classic summon monster and natures ally lines it also comes with abilities that support binding and planar ally and all of the spells you will need to make use of those options.
Coupled with the class itself there are a selection of excellent feats available only to this class that even further expand the options available to the player. These feats, and much of the spell list draw on a massive breadth of books and really showcase the amount of research that went into this class, that being said if you have limited access to a lot of 3.5 material you might find your spell list and feat choices dramatically shrunken, but still definitely enough to play with.
In closing this is precisely what a good homebrew class should be. It's specialized, fun, innovative, calls on a vast amount of material, and above all is carefully crafted in a way that you don't find even in most core products.