Mechs are pretty much a battery in a tube. From there it really depends on how you prioritize what you want. Things like:
What do you want to pay - good $20 clone mechs to high end colectors mechs. If starting off clone see how many sold of the one you like. Stay mainstrem to start.
Looks - simple and clean, scuplted, wood etc.. - Looks and fits play a lot into my decision. I like the Nemesis. Caravella and King. Much harder to match up brass toppers than stainless. The RSST fits the Nemisis stainless really clean and matches the finish pretty close -
What device you want to use may dictate the mech. An AIOS is built for Chi-Yo and fit on a small group of devices A IGO fits just about any one with a 510.
What size - batteries - Some are telescoping some have tubes I think tube look cleaner (my opnion) but telescoping are much easier when changing battery lengths. - What batteries do you have on hand may decide. If sub ohming do you battery research.
Clone or a orginal buyer -
Getting started the k-100 is probably one of the most popular first basic mod. I think from there it is the hype of the day for many.
I hate the depends answer but...it really does depend.
DO not buy into the ones hits harder than the next. There might be some small voltage drop in one vs another but in the end it is really your build that matters and what looks and feels right to you. Pick one, start low cost, see what you like about it and go from there. Trust me you will not stop at one.
TBAZ