It should be a favourite song of your special person or one which he/she can relate to easily and certainly one which is familiar to everyone present. You may need to ask around if you are not sure : please provide a couple of song choices.
I suggest sticking to a straightforward song where the verses are 4-6 lines in length and the chorus (the bit which is repeated through the song) could be another 6 lines long.
Well known traditional folk songs such as “Ilkley Moor Bar T’at” work well as the structure is straightforward and groups of people can sing it easily. Equally most pop songs lend themselves to this treatment as well: e.g. “Livin La Vida” is a fast paced song with a straightforward structure.
Your choice can be by any singer or group : The great singers of the past, Frank Sinatra, Bing Crosby, Barbara Streisand, Bob Marley as well as Adele, Emil Sandé , Ed Sheeran. Bands too such as the Smiths, the Beatles, Radiohead: any artist as long as the song can be sung karaoke style!
A happy song is probably better than a sad one but I am sure even Leonard Cohen’s lyrics can be adapted for the purpose!