With trading volume of more than 12 mil shares in one day, it looked like C31 is the most popular. It will expire in another 98 days. However, it is almost in the money. Due to the low conversion premium of 2%, your risk of investing in this instrument is almost same as investing in mother shares IF you are prepared to "exercise" the Warrants by buying mother shares as continuing exposure upon Warrants expiry.
In my opinion, C27 and C28 are even better as their conversion premium of 3.47% and 4.46% respectively are only slightly higher than C31 but their lifespan of 172 days is almost double that of C31.
C30 is unique because it has almost one year's lifespan (266 days). However, its conversion premium is 8.91%, higher than most other Warrants.
Different people have different risk appetite. I think all of them are reasonably good and worth a punt, except for C33 and C34, which has high conversion premium of more than 20%.