Only use perfectly ripe avocados. Put the flesh in a bowl, add lime, garlic, and more Kosher salt than you think you need. Mash that stuff up. Done.
I used to go crazy with add-ins/garnishes with my guac. Now, I think the best guacamole is just avocado and lime. If you don't have a fresh lime, you can get away with using bottled lime juice, but you'll never get the tangy kick lime zest adds.
Lots of people make their guacamole without garlic, and sometimes I am one of them. If you have a perfect avocado it is almost criminal to do too much to it besides adding a little salt.
Garnishing guacamole might make it look and photograph better, but it doesn't necessarily make it taste better. Some people think we first eat with our eyes, so here are a few garnishing/flavor add-in ideas: Freshly cracked black pepper, chopped cilantro, chopped tomatoes, chopped red onions, ground cumin, coriander seeds, chili powder, more lime zest, even avocado oil (like hummus has olive oil dribbled on top.), chopped jalapeno, chopped green chilies, shredded cheese ... the list is endless.