Lazy me is lazy to compile everything on a proper paragraph, so i'll just spit out everything I know:
Which crop to use:
I recommend using quick growth mid-tier plants to increase stats.
Stickreeds fit perfectly on that category, because they can grow very quick, thus crossbreeding faster. Also they'll eventually mutate into everything due ICish nature of crops.
Stats:
Thus, once you get a 21/31/N (resistance is useless for yield, dont increase it as it slows down growth for no reason) stickreed, you can keep crossbreeding them for mutations, as the mutations will inherit the stats of the parent stickreeds.
Crops will start acting like weeds (spreading weeds to adjacent blocks, but also immune to them) at growth >=24.
Alternatively, you can go for 31/31/N stats, but that may be difficult to spread due the weed spread ability of them. Manual usage (do not use cropmatron) of weed-ex is advised for such crops, but with much caution (you'll need GT scanner to watch weed-ex levels on soil. Do not increase weed-ex level beyond 75 or it will damage your crop stats! [one usage of weed-ex increases it by 50]).
Use GregTech portable scanner for in-world crop scanning as soon crops are bred (those will drop scanned bags on proper harvest aswell). Said scanner will also tell you much more other than the stats of the crop, like soil quality and products used on them.
General tips:
Make several 2x2 "nursery" plots with stickreeds being diagonal (2 breeding spots), keep harvesting them, analyze, substitute low stat ones with higher ones till you get your desired stats.
Increasing nutrient, airquality and hydration stats (of the crop field) will boost the growth of crops planted on them.
To do such:
3 dirt blocks below the farmland (+nutrient)
skylight (+airquality)
height (+airquality) [bonus caps at Y 128]
good biome (+humidity) [forest-ish does +5 and swamp/jungle does +10, bad biomes like desert and ice will actually decrease this stat]
artificial hydration (+humidity) [Usage of cropmatron is advised, especially because water is cheap]
artificial fertilizer (+nutrient) [Usage of cropmatron is advised, given you have plenty of fertilizer]