In Ice Path at Night for Crystal at level 20 range, and Route 28/Mt.Silver for G/S and level 40 range.
A great base 115 Speed, and a good 95 attack, but all his STAB moves will be using a pitiful 35 sp.attack.
When you catch him he will have Screech and Quick Attack in all versions, his most consistently powerful move is Strength, and can learn Blizzard for some power against dragon types. He learns Slash at 49. Dig can be a useful TM for some coverage.
Sneasel has a good amount of power, and could be useful as a revenge killer, but movepool and STAB hinder it.
Awful dual special type results in no physical STAB moves. A 4x weakness to fighting, and weakness' to rock, bug and fire aren't helping. However he can abuse the psychic immunity.
For Crystal he will need some grinding to go up against Clair. Sneasel also does not enjoy Victory road and the numerous wild rock types, limiting its chance of leveling. In the elite 4 Sneasel will be able to handle Will's team, but his Xatu require Blizzard and is rarely a safe move. He can't fight Bruno unless he is so over-leveled he can OHKO everyone, although Hitmonchan will Mach Punch him to death, Onix will require a lucky dig with no EQ, or a Blizzard that doesn't miss, and that isn't a situation you want. By the time you get to Lance you will need to restore Blizzard PP, Areodactyl will out-speed you unless you've heavily leveled and rock slide you to death. Charizard and Dragonite also have the potential to ruin you with Flamethrower and Fire Blast. In Gold and Silver by the time you get him at Mt.Silver your team will be well established, with no evolution there isn't any reason to train him since he can't evolve.