Hard to say what is the "best" paint. Various mediums and brands have their pros and cons. It really comes down to your preferences. It also depends on how you intend to apply the paint (brush vs. airbrush) and whether you want to deal with some of the hassles of enamels (smell, ventilation requirements, need for strong thinners, etc.).
I'll list some of the pros and cons of the main types of paint: acrylics and enamels.
Pros: Easy to clean, no strong thinner required (water is all that is needed for many), no strong smell (Tamiya have more odor than most) and generally non-toxic, readily available, allow you to use mineral spirit/turpenoid washes without sealant.
Cons: Some believe the finish is not as good as enamel, don't adhere as well as enamels, some brands don't brush as well as enamels, a little more finicky when airbrushing (tip drying and a little more difficult to spray tight patterns), don't drybrush well.
Some brands: Vallejo, Polly Scale, Tamiya, Gunze Sangyo, Model Master Acryl, Andrea
Pros: Great brushing and airbrushing qualities, broad color ranges and established color matching, better adherance, drybrush well.
Cons: Toxic, require strong thinners and plenty of ventilation, hassle to clean up, must seal before using most types of washes
Some brands: Humbrol, Model Master, Floquil
In general, acrylics dry faster than enamels. This can be a good or bad thing, depending on the circumstances.
Personally, I work mostly in acrylics. Previously, I would airbrush mostly in acrylics, but detail (brush) paint in enamels. However, acrylics have come a long way in improving their brushing qualities, so I am using them almost exclusively now (except for washes and drybrushing). A couple of lines (Vallejo and Polly Scale) brush about as well as enamels when properly thinned (distilled water works great with these two brands). I used to prefer Tamiya for airbrushing, however now that I've gotten the mixtures down for Polly Scale and Vallejo I have equal success with them and since they are water based (Tamiya is alcohol based) I prefer working with them. So, when push comes to shove, I would say my favorite paints are 1.)Polly Scale (best combination of brushing and airbrushing qualities) and 2.)Vallejo (best brushing acrylic I've ever used).
Finally, a word about primers and topcoats. I prefer Tamiya's primer (no real reason, it just works well for me). Primer may not be necessary in all instances, however. If I build a kit out of the box (no photo etch, resin, etc.) I usually only spot prime areas to check for imperfections (gun barrel mainly). One of the major benefits of priming is that it provides a uniform base for the basecoat, but this is only really critical when you have mixed media (in my opinion). Today's acrylics generally adhere well enough to not absolutely need primer. About topcoats: I don't use them if I can avoid it. I find that sheen can generally be manipulated better with other methods rather than a universal top coat. Lacquer-based dullcoats tend to yellow with age and acrylic flats can sometimes be a little finicky. Also, topcoats dull some weathering effects (particularly pastels, powders, or pigments). Finally, I prefer a little diversity in sheen on my vehicles and an overall topcoat homogonizes it.
One final bit of advice: Before you prime or paint, make sure you clean the surface well. There are plastic prep solutions on the market, but warm soapy (dishsoap) water works just fine. Some things paints don't like (especially acrylics) are grease, oils, or mold release agents. Make sure that your surface is free of these.
Well, this was longer than I intended, but I hope it helps.