HILo
Others with more experience can give you a better answer. But I use HiLo myself, and I play mainly DD. HiLo works just fine there. As you shrink the number of decks, playing efficiency becomes more significant (those points where you depart from pure basic strategy and play differently based on the count).
For example, someone recently ran a sim on a very good SD game and showed that they could beat it flat-betting using a good L2 count. Just the BS departures are enough to beat the game. With HiLo that's not going to happen. With HiLo, you are forced to spread your bets a bit as HiLo is very good at telling you when to bet big, but not as good in telling you when to break with BS and play something different. So you have to use its strength and spread your bets more. Rest assured that you can do just as well with HiLo, assuming you can get a decent spread down on the table without getting tossed. For DD, 1-4 will do pretty well, 1-8 is better.
I have done five test sessions in the past few months, starting a trip with $1000 (if I lost it all, that test was considered a bust, if I doubled the BR, the test was concluded as a success.)
Amazingly all five tests were successful. On one test I doubled my BR in about 500 hands (about 5 hours give or take an hour or two) of playing. On another test It took 5000 hands, around 50 hours, and my $1000 dropped all the way to $35 (I was spreading $5 to $40, or $5 to 2x$25 if there were other players at the table). The count was high, I bet $35 rather than $40, and won, and by hand 3000 or so I was back to dead even. Another 2000 hands saw me double.
I have tried this twice so far with $500, using the same spread. Doubled once, busted once.
As you see, HiLo can be used to beat DD with no problems. I've thought about trying a L2 system, but I have practiced HiLo for so long, I can almost count blindfolded. Learning a new set of values for the cards would probably take quite a while to squeeze all the errors out. And a few errors in a L2 count could wreck all the advantage from going to the L2 count. My concern would be a moment of inattention and I could easily mix in a hand of HiLo counts, totally wrecking things. If you are one of "those" that can use multiple counting systems without trouble, then it is worth it. A friend of mine uses HO2 for SD/DD and HiLo for 6D. And we have played DD and 6D in the same playing session. I'm afraid my head would explode. However, for revenge, I do occasionally challenge him to a few drills on CVBJ, and I can absolutely smoke 'im on speed and accuracy, because I have counted hilo enough that converting to TC is just "automatic".
I think the moral of the above is that HiLo is "good enough" for SD and up, although if you find yourself with lots of good SD games available, a L2 count would be worthwhile if your head doesn't explode...
Use the available tools before changing. CVBJ for starters. When you can play and bet for an hour, and play about 600 hands in an hour on the simulator, and not have it complain about any bets or BS departures, you can be pretty sure you can survive in a casino maintaining the count and playing according to it.
Someone told me when I was just thinking about getting started in counting, "it is far better to use a simpler count and do it accurately, than to use a more complex system and screw it up on occasion."
BTW, while it probably isn't universally true, my HO2/HiLo friend simply can't do HO2 on a shoe. The numbers get unwieldy at times. I've had RC at +60 on a shoe, a L2 count could double that, and it gets (apparently) a bit harder to keep up. I personally don't like big negative counts anyway, for the same reason even using HiLo. But before the math gets too tiring, that's a good sign to "Wong out" anyway.