What's the safest and most effective drug for lowering diastolic bp?

Difficult. Most HTN pts require at least 3 different drugs to control hypertension. The guidelines recommend to start with low dose diuretics but it is ok to use calcium channel blockers like amlodipine or beta blockers like metoprolol.( especially if they have coronary disease or atrial fibrillation). Acei's like Lisinopril are also good for CHF and in diabetics. Arbs:diovan or losartan have good profile.
Beta glucan -mushrom. Beta glucan in mushroom lowers diastolic BP - biotics has shiitake 500mg caps use 1000mg twice daily or qnlabs has reishi and maitake use 2-3 pills twice daily.
Lifestyle first. Weight loss and exercise are best, without doubt. Pharmacologic choices are many, but selection best left to physician and often constrained by insurance formularies. Fortunately, many generics available that are effective. Ace inhibitors and calcium channel blockers work well, with or without thiazide diuretics, but your specific options best left to doctor who knows your specifics.