The stock market rally of 2023 has surprised even Wall Street veterans. Here are the 10 best-performers on the S&P 500 and what could be driving them.

• The rally is US stocks this year has surprised even Wall Street veterans, following a dismal 2022.
• The S&P 500 rallied nearly 9% this year, driven mainly by expectations of cooling inflation.

After US stocks' dismal performance in 2022, many Wall Street experts expected more pain in the market this year. But they've performed so well in the early going that even longtime investors have been taken aback.

"I think the upside surprised a lot of veteran traders, including a few old fogies like me," trading legend and UBS floor director Art Cashin told CNBC.

January is a month when investors typically stand down from high-risk assets, so the optimism in the market was "odd to see," according to Wells Fargo equity strategist Anna Han.

The benchmark S&P 500 US stock index has rallied over 6% in 2023 so far. It's risen mainly thanks to expectations that cooling inflation will allow the Federal Reserve to pull back on its aggressive interest-rate hikes, which typically weigh on asset prices and economic growth.

Here are the best-performing shares on the S&P 500 this year as of Thursday's close, and what could be contributing to their rise.

1. Tesla is leading the pack with a 68.3% gain, as investors warm again to tech stocks. The Elon Musk-led electric-vehicle maker has enjoyed strong fourth-quarter results, news of an expanded EV tax credit, and a jump in sales in China as price cuts spur demand.

2. Catalent has risen 59.9%, thanks in part to reports that life sciences giant Danaher is interested in taking over the New Jersey drugmaker. It beat revenue expectations though it missed on earnings in its quarterly earnings this month.

3. Nvidia has gained 52.9%. The graphics chip specialist is well placed to benefit from the ChatGPT craze, given its tech is found in devices that run artificial-intelligence tools and it could play a crucial role in AI development. Investors keen to get in on the emerging tech may have been taking note.

4. Align Technology, the leading maker of Invisalign teeth straighteners, is up 52.4%. Shares jumped 27% after it reported better-than-expected fourth-quarter earnings in February. Some analysts believe pent-up demand in a reopening China could boost the company's shipments this year.

5. Warner Bros. Discovery has gained 51.6%. It decided in early February to abandon its plans to consolidate its streaming services and keep Discovery+ as a standalone platform. The strategy shift was meant to avoid losing subscribers who might not want to pay premium prices to watch other content.

6. Royal Caribbean has advanced 48.8%. Some analysts expect a more-upbeat year for cruise stocks as pent-up demand from the pandemic plays out.

7. Meta Platforms has moved up 47.9%. The Facebook parent's stock is having its best run in a decade after CEO Mark Zuckerberg hailed 2023 as the "year of efficiency" with a round of cost cuts and share buybacks.

8. Monolithic Power Systems has gained 44.4%. The chipmaker's stock already rising in 2023, and rallied 10% Wednesday after its fourth-quarter earnings beat Wall Street expectations and it increased its quarterly dividend.

9. Carnival has risen 43.1%. Like others in the cruise and hotel industry, it's seen as potentially benefiting from consumer spending on experiences this year.

10. Norwegian Cruise Line has seen a 38.2% gain.