Take nothing away from Habana, he has been a brilliant servant of the game and has had some amazing highs as well as some very mediocre seasons.
To compare him to Lomu is perhaps a stretch. Lomu's 15 tries come off just 10 games and 2 WC's. (Including 4 tries against England In the 1995 semi)
This is Habana's 3rd WC and I'm sure he has played far more than 10 matches to achieve his 15 tries.
In addressing who is the greatest I think it depends on the era. The game has evolved so much and especially in the pro era. I would rate a few players for this accolade. Keeping in mind that they each had different strengths that set them apart. Os du Rand, Joost vd Westhuizen, Mark Andrews, Andre Joubert, John Smit.
Then there is the cold years (1970's and 80's) when SA didn't get to participate fully on the international scene. There were some very talented players in that era who just never got the opportunities yet were giants of their time. Uli Schmidt, Danie Gerber, Rob Louw, Louis Moolman, Theuns Stofberg, Moaner V Heerden, Gerrie Germishuys, Morne du Plessis, Piet Visagie, Jan Ellis, Hannes Marais.
And the there is the unluckiest non-springbok of all time - Thys Lourens.