So I believe very strongly that Selesnya is going to be your best bet if you want to almost always have a solid pool to build from. My reasoning is this:
Most of selesnya's pool involve cards that are independently good. This means that if your board is being controlled, each creature you have in your arsenal will likely have a bigger impact than something like golgari which rely on creatures in your graveyard, or mentor which requires you to have a board state established. Additionally, this guild has several very reliable bombs that are extremely difficult to deal with that can be relevant in bot mid game and end game such as the convoke 7/5 hexproof. Lastly, bot green and white have solid contending guilds, which make splashing in another color to make up for what you are missing, very doable. Dimir boxes that splash izzet will struggle with the aggro of boris and the sheer numbers of selesnya and dimir splashing green will likely have a pretty split time as the mechanics play beautifully with each other, but then you have to rely on two of the largest card pools in the set, which means you have to deal with a solid possibility of getting nothing of real use outside of your seeded pack.
To support my stance the top 5 in our 50-man event last night had 3 selesnya, 1 boris, and 1 dimir. Both boris and dimir splashed colors... selesnya all went full guild.
Most of selesnya's pool involve cards that are independently good. This means that if your board is being controlled, each creature you have in your arsenal will likely have a bigger impact than something like golgari which rely on creatures in your graveyard, or mentor which requires you to have a board state established. Additionally, this guild has several very reliable bombs that are extremely difficult to deal with that can be relevant in bot mid game and end game such as the convoke 7/5 hexproof. Lastly, bot green and white have solid contending guilds, which make splashing in another color to make up for what you are missing, very doable. Dimir boxes that splash izzet will struggle with the aggro of boris and the sheer numbers of selesnya and dimir splashing green will likely have a pretty split time as the mechanics play beautifully with each other, but then you have to rely on two of the largest card pools in the set, which means you have to deal with a solid possibility of getting nothing of real use outside of your seeded pack.
To support my stance the top 5 in our 50-man event last night had 3 selesnya, 1 boris, and 1 dimir. Both boris and dimir splashed colors... selesnya all went full guild.