These two guys take people down at will and the two best offensive wrestlers in the UFC right now. In the past the best have been the likes of GSP, Brock and Cain.
These two have the highest takedown average per 15 minutes I can see among any fighter ever in UFC history (Khabib with 6.15 and Covington with 5.93 per 15mins and takedown accuracies of 48% and 53% respectively).
In his 8 UFC fights Khabib has had 39 takedowns and in Colby's 8 UFC fights prior to the Maia fight (where he showed his defensive wrestling) he had 41 takedowns.
Both are decorated wrestlers in their own right. Colby is a former 174lb Division 1 All American/5th placed finisher in 2011 and Khabib is a former two-time Combat Sambo World Champion.
Don't respond to this by saying there are more accomplished wrestlers out there i.e. Yoel Romero and DC because I'm speaking about the skill set of offensive wrestling as it is utilised in MMA.
Khabib is the better all round grappler (who can also execute judo throws) with the better ground game but they are very close when it comes to takedowns.
Colby is seems to be a dickhead but you can't discredit his ability
all stats from:
http://www.fightmetric.com
http://www.fightmetric.com/fighter-details/dc9572dd6ec74859
http://www.fightmetric.com/fighter-details/032cc3922d871c7f