Berardi recommends as a minimum: (from his grappler nutrition book)
1) A protein blend such as Metabolic Drive
2) A recovery drink such as Surge recover
4) Greens type drink, go with Superfood
5) fish oil, go with Flameout
I believe these are a must for any athelete altough 1 3 4 can be ommitted if you are getting enough from your diet which is very difficult to do for most. Noobs please remember they are supplements and not replace a good diet.
OP asked for 1 based on budget. OP what is your budget?
Vitamin D is one of the cheapest there is, checkout the vitamin D thread http://www.T-Nation.com/free_online_forum/diet_performance_nutrition_supplements/vitamin_d_1
Just to repeat Creatine, Leucine and ZMA are ridiculously cheap.