First of all it really depends on personal preferences. Some people prefer to have an initiative, other prefer to have a pawn up even if they'll have a tough game ahead.
I am extremely materialistic, so I would never play a gambit.
Having said that, the Smith-Morra is probably the best gambit. In my sicilian I get under serious pressure because I can't keep my queen on c7.
I have a friend with whom I play chess often, and he starts off with opening gambits that I've never seen in my life before. He loves open games and therefore employs these gambits in a lot of his games. Sometimes he wins, and sometimes I do, but in grandmaster games are they played often? I've done some research and the gambits he play are:
The Blackmar Gambit:
The Smith-Morra Gambit:
The Blackburne Shilling Gambit:
Evan's Gambit (I've seen this one before):
Are they sound? How does one play against them?