Ice types are very good offensively, demonstrated by the fact that 3 of Cynthia's Pokemon are weak to it. However, they have 4 weaknesses and only resist themselves. As a result, ice Pokemon suck defensively and Pokemon that are part ice type tend to have a lot of weaknesses. Because of this, ice Pokemon generally make better sweepers than tanks.
Abomasnow: Hits both physically and specially, but not very hard in either. It has a lot of weaknesses and it's slow on top of all of that. Ice shard does fix that somewhat.
Weavile: Outspeeds just about everything and gets good STAB from night slash and ice punch (route 112 tutor). All of it's power is concentrated on speed and physical attack, which is pretty much all you need.
Glaceon: Relatively bulky with a lot of special attack. It doesn't learn a good STAB attack by level up, but you can get a TM13 ice beam by buying coins in Veilstone City. It only has one type, however.
Mamoswine: I'm not really sure, but I think Mamoswine gets the most super effective!ness from its STAB attacks of all Pokemon. It tends to be both slow and not defensive, so I recommend focus sash and ice shard.
Glalie: Being well rounded (no pun intended) can sometimes be a bad thing. It's not very fast, bulky, or powerful compared to your other options, and it only has one type.
Froslass: Although it's attack stats are the same, it's only good moves are special. It can outspeed almost everything except Weavile.
I'd say Weavile is the best (it's the one I used, and it outsped everything and OHKO'd them with ice punch), but Mamoswine and Froslass are good too depending on the rest of your team.
