The advice given about eating right and exercise is great, but seeing that you asked for supplements, here are a few. Cinnamon helps regulate blood sugar and if you are diabetic, Mr. Happy will be Mr. Wimpy. If you blood pressure is kinda high, you might want to moderate it with some garlic and anything for prostate health like saw palmetto and a very other things are good. As well, watch the hard alcohol if you drink and smoking.

Meeeeeeeeeeeeeeeeeeeeeeeeeeeeeee!!!!!

You can put me over a counter and I will most certainly keep my Mister Happy.

(Y'all knew that was coming)

There's lots of reasons that Mr Happy may not be happy.  

If you're diabetic vs. hypertensive vs. you have MS vs. you have vascular disease - it will alter what the best supplement is.

I'm diabetic, and do have some issues with ED.  For me it's important to take Vit B and D supplements to keep Mr. Happy happy, in addition to managing my blood sugar.  For other people it might be a totally different supplement.

Arginine is good for most people unless you've got liver or renal disease.  Just about any supplement will be fine for most people but could be lethal for a particular group.

Even with supplements, check with your doctor !