EHpro and Hcigar are consistently the best quality clones. Regardless of the model, you can't go wrong with either one.
Tobeco clones are generally very good, but they do have some quality control issues at times. It's usually best to wait until they've released a new revision or two of a product before buying one.
Cheaper Fastech clones are cheaper for a reason. You see a few popup in this thread. Crunchy buttons on a Nemesis? The only time I've seen that on an Hcigar or EHpro is when the user didn't get the button pin tight and it wobbled around. If that pin is nice and tight it's butter smooth every single time, no magnets needed.
And are you really saving money on that cheap fasttech clone if you then have to spend more money on magnets to make it work right?
Same story with the Kayfun clones. Hcigar and EHpro are as close as you can get to the real thing. Fasttech ones are hit and miss with poor quality control leading to issues like the juice channels either being blocked or too open, leaking, etc...
Now, in regards to mech mods I would first consider what you are wanting to use it for. If you're looking for a good, reliable, high powered vape and maybe dabble a bit in subohming just to see what it's all about then any good quality mech mod will do the job.
However, if you're going to be serious about subohming and every last little bit of power counts then you're going to want to look at the full load voltage loss of the mech mod in question. Some mods lose a LOT of power through the body of the mod, others are extremely conductive and loose almost nothing. All brass or all copper mods with silver plated connections would be ideal, but some stainless steel mods also loose very little if any voltage through the body. A loss of just .1-.2 volts in the body can mean a loss of 3-5 watts at the coil when subohming.