Anything that involves never being on PS2. Make friends. Work as a team. Your certs won't even come into it. If you really must, I'd recommend certing your infantry out most, as you'll get the highest ROI as infantry>vehicles in the current meta. Lightning/ESF are the obvious choices, but MBT/Lib/Harasser will do you just as well - randoms very rarely make -terrible- gunners. They're not great, but they'll do...
IF I used vehicles, I'd cert up the Lightning, though its near impossible to drive those things around in any useful way these days with all the map clutter. When one can drive really fast and hit people on the move, it can be a lot of fun, but many of my rampages are just stopped by terrain. In BF2 there were amazing tank battles, because you could actually move about the battlefield, without snagging on tons of garbage in the way. SOE could learn a lesson from these tank battles, that came late in BF2's existence, but breathed a new life into it.
I tend to solo quite a bit, and I find that ESF or MBT is the best option, but that is with a huge asterisk. The only play style that is really viable for a solo tank driver is if you are TR and can snipe other tanks from afar, with AP and lockdown, because if you go in close combat you will have a big disadvantage to 2/2 tanks.
You can solo as a Vanguard or Prowler decently, but you have to play very conservatively. Also leave your secondary gun open to randoms, they'll help you more than hurt. You should never ever run a 1/2 Magrider. Ideally, it would be the ESF, but the learning curve is steep and getting steeper, so pick this only if you're willing to dedicate yourself to it. And the weapons are fairly pricey too, outside of the AI noseguns and the rotaries. The Lightning is a toss-up. It can serve multiple roles, but you need to have the guns for each, so it's a massive cert investment or you cough up the dough. The HEAT is a cheap start if you're saving certs, but only with the dedicated guns can you maximize the performance of the Lightning.
ESF is an obvious choice, though flying without a wingman is much different than soloing on the ground where you can hide in the middle of friendlies. Lightning is a fairly serviceable option as well. It's what I use most on my main and I'm always solo - haven't been in an outfit in over half a year. Harasser isn't actually half bad 1/3 depending on the gun you put on the top, and if you pick up a random gunner - all the better. Use one on an alt - with the AI option it feels around as effective as an AI Lightning when 1/3, but with a different style (worse at range than a tank, but better equipped to survive getting close). MBTs are fun, but are too resource intensive to focus on as a solo player for most people. Unless you're extremely skilled a Lib requires a second person. A Galaxy will probably want at least 2 others to man the guns on top of the pilot, so not really a viable solo option. Flash is fun and can be useful, but is a bit too fragile (you will be easily shot off long before the Flash would've exploded a ton).
