"Fast", "high Defense", and "high Special Defense" were not defined, so I will use my own interpretation.
According to this answer, the average Speed stat is 70, the average Defense stat is 75, and the average Special Defense stat is 72. Therefore, a Pokemon that fits these criteria might be one with stats higher than these.
/nds grass, solarbeam, sunnyday, spe>70, def>75, spd>72, all:
Arceus-Grass, Calyrex, Celebi, Gourgeist, Leavanny, Meganium, Ogerpon, Ogerpon-Cornerstone, Ogerpon-Hearthflame, Ogerpon-Wellspring, Serperior, Shaymin, Tapu Bulu, Tsareena, Venusaur, Whimsicott, Zarude
This doesn't mean much in a competitive context, where a "good stat" varies wildly based on tiers. I think the most consistently usable Pokemon in this list are Celebi, the Ogerpon forms, Serperior, and Venusaur; of these, Serperior is the fastest while Venusaur can use Chlorophyll to double its Speed. Whimsicott is also a powerful choice in Doubles formats.
The moral of the story is that you should check the viability rankings of the tier you're playing and see which Pokemon best fits your needs. Typically Solar Beam + Sunny Day is a poor combination, so I would also recommend checking the Smogon analysis page for your Pokemon's recommended set(s).
