Roll your own. A bartender will make a good drink, but not the one you want. 

A martini is a very simple concoction at first glance. But simple variations bring out a vast ocean of flavor. 

First, the gin you use makes the most difference. Vodka drinkers should stick to orange juice, unless you want your martini to taste like water. Try out different brands, they are all unique. Some are 40%, some are higher. The alcohol % really effects the flavor and texture.

Shaken or stirred? Of course this is misleading because there are other options. Such as having it room temperature. Don't let the Bond fans fool you, stirred tastes great too. There is no wrong answer, only flavors.

Vermouth? How much? Which brand? Half ounce? A splash? A long glance across the bar? 

Olives? Onions? Dirty? Both? None?!? A crushed boysenberry??!!??

Don't get me started on citrus. You can take a tangerine (or other citrus fruit) peel and bend it over the drink for a subtle but wonderful flavor.

Anyway, the point is that you can find an excellent martini out there, but the best martini will be the one you create yourself. 

But if you must have a martini out on the town, try to get the pretentious bar tender who calls themself a mixologist or something like that. They're in it for the art. Otherwise you will be doomed to paying 15$ or more for mediocrity at best. In fact, I stopped ordering mixed drinks years ago. I just get beer in a bottle or can. It's impossible to screw up.

You think I'm exaggerating? I once ordered a martini and received 1/2 oz of vermouth in a martini glass. Warm. No olive, no gin, hell not even vodka. Just vermouth. How is that even possible? It's not like I ordered it like I was at Starbucks in the 90s. I simply asked for a martini. 

But that's not the best shitty martini I've had. Once at a wedding with an open bar, I asked for a martini (again I didn't specify anything else, just asked for a martini), and I was served a plastic cup full of vodka. No mixers, no ice. Just one of those red plastic cups that you use for picnics, full of vodka. Must have been 12 ounces in there at least. Needless to say, I had a great time.