Which altcoins are most vulnerable when $BTC is selling off?

I would divide the market into three groups.

Group A: Relatively established altcoins

Examples include:

Ethereum (ETH)
BNB (BNB)
Solana (SOL)
XRP (XRP)

These assets are not safe from major declines. However, their liquidity and established market presence can make them more resilient than many smaller tokens in some market conditions.

My approach: Monitor BTC support, each coin's own chart and relative strength before considering a purchase.

Group B: Mid-cap altcoins

Examples include:

NEAR Protocol (NEAR)
Optimism (OP)
Arbitrum (ARB)
Sui (SUI)

These coins can rebound strongly when market sentiment improves. However, they can also experience deeper corrections when capital leaves the altcoin market.

My approach: Avoid buying simply because the price has fallen 10% or 20%. Wait for evidence that selling pressure is weakening.

Group C: Small-cap and highly speculative altcoins

This group includes many low-liquidity tokens and meme coins.

These are the coins I would approach most cautiously during a Bitcoin sell-off. They may suffer from thin order books, concentrated ownership, token unlocks and a lack of consistent buyers.