You can't go wrong with a set of solid colors. They're good for incorporating other gels like toppers or chrome (see the misc. section for some examples!), are perfect for nude looks, and are overall a good pick for minimalists.
Honestly, I'm a sparkle girl. If my nails don't have at least a hint of glitter then I'm not fully happy with the look. Here are some of my favorite ones that don't skimp on the sparkly factor.
These sets have a little bit of everything! If you're just starting out I recommend selecting one of these, since you can mix the colors to get additional pigments--or you can create different hues by mixing in black, white, or clear topcoat polishes...
For all other gels that aren't sparkly or solid. These gels are great for diversifying your collection without breaking the bank, since they can help elevate the look of the colors they're paired with.