First off, Jolteon has Volt Absorb and gives it an Electric immunity. But I still choose Mamoswine. It has great attack, and decent speed. Mamo also has a good HP stat and and has some decent abilities in Oblivious, so it can't fall in love, and Snow Cloak so it will have raised evasion in Hail.
Now, Lapras doesn't have the best stats for in-game, they are more defensive. It only has 85 offenses, which aren't really ideal. It can be gotten earlier, so I highly recommend using Lapras and using it until you get Mamoswine.
This is the moveset I recommend running in-game for Mamoswine.
Mamoswine @ Leftovers / Sitrus Berry / Lum Berry
Trait: Oblivious / Snow Cloak
Adamant / Jolly Nature
- Ice Fang
- Stone Edge / Rock Slide
- Return / Filler
Here's one for Lapras before you get Mamo.
Lapras @ Leftovers / Sitrus Berry / Lum Berry
Trait: Water Absorb
- Ice Beam
Hope I helped! :)